the potential of accelerator secondary ion mass...
TRANSCRIPT
DISS. ETH NO. 15046
The Potential of Accelerator Secondary Ion Mass Spectrometry
in Environmental Sciences
A dissertation submitted to the
SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH
for the degree of
DOCTOR OF NATURAL SCIENCES
presented by
COLIN MADEN
Dipl. Phys. ETH Zürich born May 4th, 1973
citizen of Riniken, AG
accepted on the recommendation of
Prof. Dr. R. Eichler, examiner Prof. Dr. M. Suter, co-examiner
Prof. Dr. W. Kutschera, co-examiner
February, 2003
I
Contents
Abstract__ __________________________________________________________ 1
Kurzfassung ________________________________________________________ 3
Chapter 1 Introduction ___________________________________________ 5
1.1 SIMS and AMS and their Incommodities_______________________________ 6 1.1.1 SIMS ________________________________________________________________ 6 1.1.2 AMS ________________________________________________________________ 6
1.2 History of Accelerator SIMS _________________________________________ 7
1.3 Outline of the Thesis ________________________________________________ 8
Chapter 2 Experimental Setup ____________________________________ 11
2.1 Summary of the Accelerator SIMS Facility ____________________________ 11
2.2 The Caesium Source _______________________________________________ 12
2.3 The Sputter Chamber______________________________________________ 14
2.4 The Low-Energy Mass Spectrometer _________________________________ 16
2.5 The Accelerator and High-Energy Mass Spectrometer___________________ 17
2.6 Particle Identification ______________________________________________ 18
2.7 Data Acquisition __________________________________________________ 21
2.8 Figures of Merit of Accelerator SIMS_________________________________ 22
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix ________________ 27
3.1 Formalisms ______________________________________________________ 27 3.1.1 Sputter yields_________________________________________________________ 28 3.1.2 Secondary ion currents _________________________________________________ 29 3.1.3 Quantification formalisms_______________________________________________ 31 3.1.4 Parameters influencing secondary ion yields ________________________________ 32
3.2 Negative Secondary Ions from a SiO2 Matrix with a Cs+ Primary Beam ____ 35 3.2.1 Samples and implantation technique_______________________________________ 35 3.2.2 Accelerator SIMS results________________________________________________ 38
Chapter 4 Platinum Group Elements at the KT-Boundary ______________ 49
4.1 Introduction to the KT-Boundary ____________________________________ 49
4.2 Sample Description ________________________________________________ 50
4.3 Measurements and Results__________________________________________ 52
4.4 Conclusions ______________________________________________________ 57
II
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios _____________ 59
5.1 Introduction______________________________________________________ 59
5.2 Sample Preparation and Loading ____________________________________ 60
5.3 Instrumentation___________________________________________________ 63
5.4 Results and Discussion of first Measurements __________________________ 67
5.5 Conclusions ______________________________________________________ 70
Chapter 6 Comparison with other Analytical Techniques_______________ 73
6.1 Detection Limit and Lateral Resolution _______________________________ 73
6.2 Analysis of Small Samples __________________________________________ 75
6.3 Improvements to Accelerator SIMS __________________________________ 78
Chapter 7 Final Conclusions _____________________________________ 83
Appendix A Beryllium Chemistry ___________________________________ 87
Appendix B Results of the Carrier-free 10Be Measurements______________ 89
References _________________________________________________________ 91
Thank you! Merci! Muchas Gracias! ___________________________________ 99
Curriculum Vitae __________________________________________________ 101
1
Abstract
Accelerator Secondary Ion Mass Spectrometry (Accelerator SIMS) originated out of
the combination of conventional Secondary Ion Mass Spectrometry (SIMS) with
Accelerator Mass Spectrometry (AMS). This has the advantage that interfering
molecules in the mass spectrum of SIMS can be destroyed in the accelerator and the
fragments are separated in a second mass spectrometer.
The aim of this thesis is to demonstrate the potential of Accelerator SIMS with respect
to applications in the environmental sciences. Analytical procedures to reliably
measure trace elements over the entire periodic table with AMS have been developed.
In addition, ways to analyse electrically insulating sample matrices have been found,
as these are commonly encountered when analysing environmental samples. Because
of the limitation of Accelerator SIMS to the analysis of negative secondary ions,
negative ion yields of trace elements from silicon dioxide have been investigated. And
finally, the limits to the analysis of small samples of only 100 ng total sample size
have been explored.
The potential of Accelerator SIMS is demonstrated by applying the technique to two
applications in the environmental sciences. The results of these measurements show
that Accelerator SIMS is capable of analysing trace element concentrations and
isotopic ratios in regimes not accessible to any other analytical method presently
available. The results also reproduce measurements performed with other analytical
techniques nicely demonstrating the reliability of Accelerator SIMS.
The first application that is discussed is in-situ bulk concentration analysis of iridium
in sedimentary layers around the Cretaceous-Tertiary transition (KT-boundary).
Neutron activation analyses (NAA) revealed that the concentration of iridium is
higher in the transition horizon (~56 ng/g) than in the neighbouring sedimentary
layers of the Tertiary and the Cretaceous (~0.4 ng/g). This was possibly caused by a
meteorite impact 65 million years ago. With Accelerator SIMS it was possible to
reproduce the NAA results, but with a lateral resolution orders of magnitude higher
(~100 µm compared to ~1 cm) enabling trace element analysis in sedimentary layers
at a much smaller lateral scale. Presently, no other technique is capable of performing
bulk concentration analysis with such low trace element concentrations and with such
a high lateral resolution.
Abstract
2
The second application that is addressed is the direct measurement of natural 10Be/9Be
ratios in samples from ferromanganese crusts (carrier-free 10Be AMS). For this
application isotopic ratios of 10-10 have to be measured in samples with a total size of
only 100 ng. This alone is a unique achievement in the field of AMS. The natural 10Be/9Be ratio as a function of depth in a ferromanganese crust gives important
information on the growth rate of the crust and is used to date oceanographic events
manifested by the concentrations of other radiogenic isotopes. To date, this
information has been gained with either the direct measurement of the natural 10Be/9Be ratio with a SIMS method or with separate measurements of the absolute 10Be and 9Be concentrations with AMS and Inductively Coupled Plasma Mass
Spectrometry (ICP-MS) respectively. Accelerator SIMS results not only reproduce
previous measurements very nicely, but the method is also more sensitive and
measures to a higher precision than the already existing techniques. The new
technique can therefore determine time scales in ferromanganese crusts with a higher
precision further into the past than the previously existing methods.
Finally, the potential and capabilities of Accelerator SIMS are compared to other
analytical techniques presently available. In particular, the method is compared to
conventional SIMS and ICP-MS. At present, these are the two state-of-the-art
analytical methods. It has been shown that Accelerator SIMS has unique features and
some prospects of how these can be exploited in future are presented.
3
Kurzfassung
Beschleuniger-Sekundärionen-Massenspektrometrie (Beschleuniger-SIMS oder
Accelerator SIMS) stammt aus der Erweiterung von herkömmlichem Sekundärionen-
Massenspektrometrie (SIMS) mit Beschleuniger-Massenspektrometrie (BMS oder in
Englisch AMS). Dies hat den Vorteil, dass die interferierenden Moleküle im
Beschleuniger aufgebrochen und ihre Fragmente in einem zweiten
Massenspektrometer vom Spurenelement separiert werden können.
Das Ziel dieser Arbeit ist das Potential der Methode bezüglich Anwendungen in den
Umweltnaturwissenschaften zu demonstrieren. Dazu wurden AMS-Techniken für die
Analyse von Spurenelementen aus dem ganzen Periodensystem entwickelt.
Prozeduren, mit denen man elektrisch isolierende Proben analysieren kann, wurden
entwickelt, da Proben aus den Umweltnaturwissenschaften häufig diese Eigenschaft
aufweisen. Wegen der Eigenschaft von Beschleuniger-SIMS nur die negativen
Sekundärionen analysieren zu können, wurde die negative Ionenausbeute von
Spurenelementen von einer Siliziumdioxid Matrix untersucht. Und schliesslich wurde
die Analyse von kleinsten Probenmengen von nur 100 ng erkundet.
Das Potential von Beschleuniger-SIMS wird dadurch demonstriert, dass die Methode
auf zwei Anwendungen aus den Erdwissenschaften angewandt wird. Die Ergebnisse
werden zeigen, dass Beschleuniger-SIMS in der Lage ist Spurenelemente einer
tieferen Konzentrationen und mit einer besseren örtlichen Auflösung zu analysieren
als jede andere momentan verfügbare Analysetechnik. Die Messergebnisse
reproduzieren die Ergebnisse anderer Methoden sehr schön und beweisen damit die
Zuverlässigkeit von Beschleuniger-SIMS.
Die erste Anwendung, welche vorgestellt wird, ist die Analyse von
Iridiumkonzentrationen in Sedimentschichten vom Übergang von der Kreide ins
Tertiär (KT-Grenze). Analysen mit Neutronen Aktivierung haben gezeigt, dass die
Iridiumkonzentration in der Grenzschicht viel höher ist (~56 ng/g) als in den
benachbarten Schichten der Kreide und des Tertiärs (~0.4 ng/g). Dies wurde
vermutlich durch den Einschlag eines Meteoriten vor 65 Millionen Jahren erzeugt.
Mit Beschleuniger-SIMS war es möglich die Ergebnisse der Neutronen Aktivierungs
Analysen zu reproduzieren, aber mit einer örtlichen Auflösung, die um
Grössenordnungen besser ist (~100 µm anstatt ~1 cm). Zur Zeit kann mit keiner
anderen Analysemethode so geringe Spurenelementkonzentrationen mit einer solch
Kurzfassung
4
hohen örtlichen Auflösung nachgewiesen werden. Die örtliche Verteilung von
Spurenelementen in Sedimentschichten kann also viel genauer erfolgen, und Effekte,
welche sich über kleinere Distanzen abspielen, können untersucht werden.
Die zweite Anwendung, welche angesprochen wird, ist die direkte Messung von
natürlichen 10Be/9Be-Verhältnissen in Eisen-Mangankrusten (trägerfreies 10Be-AMS).
Für diese Anwendung müssen Isotopenverhältnisse von 10-10 in Probenmengen von
lediglich 100 ng gemessen werden. Dies alleine ist schon eine einzigartige Leistung
auf dem Gebiet der Beschleuniger-Massenspektrometrie. Das natürliche 10Be/9Be-
Verhältniss als Funktion der Tiefe in einer Eisen-Mangankruste gibt wichtige
Auskunft über die Wachstumsgeschwindigkeit der Kruste und kann zur Datierung von
ozeanologischen Ereignissen benutzt werden, welche durch die
Konzentrationsverteilungen anderer Radionuklide in der Kruste manifestiert werden.
Bisher wurde diese Information entweder durch die direkte Messung des natürlichen 10Be/9Be-Verhältnisses mit einer SIMS-Methode oder durch zwei separate Messungen
der absoluten 10Be- und 9Be-Konzentrationen mit 10Be-AMS respektive mit ICP-MS
(Induktiv-gekoppeltes-Plasma Massenspektrometrie) gewonnen. Es wird gezeigt, dass
die neue Beschleuniger-SIMS-Methode nicht nur die Messungen der anderen
Methoden sehr schön reproduzieren kann, sondern dass sie auch empfindlicher und
präziser messen kann als die bisherigen Methoden. Somit können Datierungen an
Eisen-Mangankrusten genauer und weiter in die Vergangenheit gemacht werden, als
es mit den bisherigen Methoden möglich war.
Zuletzt wird das Potential und die Möglichkeiten von Beschleuniger-SIMS mit denen
anderer Analysemethoden, die heute zur Verfügung stehen, verglichen. Insbesondere
wird ein Vergleich mit herkömmlichem SIMS und ICP-MS gemacht, da diese heute
die vielversprechendsten Analysemethoden sind. Es wird gezeigt, dass Beschleuniger-
SIMS einzigartige Eigenschaften hat und mögliche Zukunftsperspektiven für die
Methode werden vorgestellt.
5
Chapter 1 Introduction
The discovery and exploration of the structure of the atom in the first half of the last
century triggered the invention of a number of analytical methods to analyse the
composition of samples. Over the last fifty years, the available methods have been
steadily improved. Today, research, development and manufacturing control could not
be imagined without the possibilities of trace element analysis available. Many
applications in material science, environmental sciences, and other fields of research
rely on the accurate determination of the concentrations of trace elements and their
isotopic ratios in samples. But the more progress science makes, the greater the
demand for better detection limits and higher lateral resolution gets. The development
of new, more sensitive analytical techniques is therefore an important prerequisite for
future discoveries in all fields of research.
Accelerator Secondary Ion Mass Spectrometry (Accelerator SIMS) is a recent
development. As one can assume from its name, it evolved from the combination of
conventional Secondary Ion Mass Spectrometry (SIMS) with Accelerator Mass
Spectrometry (AMS). The idea was to suppress molecular interferences in the mass
spectrum of conventional SIMS by extending it with an accelerator mass
spectrometer. In the nineties, first dedicated Accelerator SIMS facilities were built
and tested by analysing trace elements in silicon wafers. Improvements of detection
limits by up to two orders of magnitude in comparison to conventional SIMS were
observed (Ender et al., 1997a+b+d; Massonet, 1998). Picking up the development of
the method from there, the aim of this thesis is to explore the limits of Accelerator
SIMS with respect to applications in environmental sciences. It will be demonstrated
that Accelerator SIMS has unique features and can perform measurements that no
other analytical technique is capable of performing. These features will be compared
with the capabilities of other analytical techniques presently available, and prospects
for future developments of Accelerator SIMS will be presented.
In this chapter the motivation leading to this thesis will be explained in more detail
starting with a short historical review of the development of Accelerator SIMS
together with work that has been performed in this field. Following this, the outline of
the thesis will be presented.
Chapter 1 Introduction
6
1.1 SIMS and AMS and their Incommodities
1.1.1 SIMS
In first experiments with ion beams in the thirties it was observed that during
bombardment with a primary ion beam secondary ions of the target material itself are
emitted out of the surface of a solid. Extracting these secondary ions to a beam and
performing mass spectrometry with them gave birth to a method that is now known as
Secondary Ion Mass Spectrometry (SIMS). By scanning the primary beam over the
sample surface the concentration distribution of a trace element on the surface can be
analysed. Also, the three dimensional distribution of a trace element can be measured
by eroding the sample layer by layer with the primary beam.
The first dedicated SIMS apparatus was built during a NASA project in the beginning
of the sixties. It was built for the analysis of lunar rocks from sample return missions.
Due to the ability of the method to produce information on the three dimensional
distribution of a trace element in a sample and also to its low detection limits, SIMS
soon found a large range of applications in various fields of research and industry.
Driven by its own success and the demand for lower detection limits and better lateral
resolution the method developed rapidly. Today, state-of-the-art SIMS machines are
highly sophisticated devices with - in favourable cases - trace element detection limits
in the sub-ppb range and with a lateral resolution of about 30 µm. Reducing
sensitivity, mainly due to reducing the intensity of the primary beam, lateral
resolutions of about 50 nm have been achieved (de Chambost et al.,1993; Cameca,
2003).
However, in spite of the high mass resolution some particles sputtered from the
sample have a mass that is almost identical to the mass of the trace element of interest.
For example: When wanting to analyse 56Fe in a silicon wafer the interfering molecule 28Si2 has almost the same mass (M/∆M = 5600). In addition the intensity of the
molecular interference is orders of magnitude higher than that of the trace element.
Even with a mass spectrometer with a high mass resolution it is difficult to separate
this interference from the iron isotope.
1.1.2 AMS
Accelerator Mass Spectrometry is a routine method to detect extremely small amounts
of long lived radioisotopes such as 10Be, 14C, 26Al, 36Cl, 129I, and others (Wölfli, 1987)
in samples of a few hundred microgram. For example radiocarbon isotopic ratios of
10-14 can be measured to a precision of ~1%. This is only possible with an efficient
Chapter 1 Introduction
7
suppression of molecular and isobaric interferences, which is done by acceleration of
negative secondary ions with a tandem accelerator after a first mass analysis has been
performed. In the so-called stripper medium (Ar gas or a carbon foil) located at the
high-voltage terminal of the accelerator, electrons are stripped off the ions changing
their charge state from negative to positive. All molecules in charge states higher than
2+ are unstable and disintegrate (Weathers et al., 1991). If the trace element is
analysed in a charge state higher than 2+ then the atomic fragments of the interfering
molecules are separated from the beam in a second mass filter following the
accelerator. In addition, the higher energy gained by the acceleration of the ions can
be exploited to suppress isobaric interferences in an appropriate detection system (gas
ionisation chamber, TOF, gas-filled magnet, etc.). Another advantage of the higher
beam energy is that cross sections of scattering processes off residual gas atoms,
which can cause a background in the detector, are smaller.
In spite of the enormous suppression of interferences, AMS is not suitable for the
analysis of stable trace elements. When analysing radioisotopes of low abundance,
contamination resulting from the analysing instrument itself is usually not a problem.
This is not the case when analysing stable trace elements where a lot more
possibilities of contamination of the sample are possible. Potential sources of
contamination are impurities in the primary beam, the quality of the residual gas in the
system, and sputtering of electrodes that are exposed to the ion beam. The use of a
dedicated ion source is therefore unavoidable in order to perform stable trace element
analysis with AMS.
1.2 History of Accelerator SIMS
The use of AMS technology to analyse stable trace elements, i.e. the principles of
Accelerator SIMS, was suggested by K. Purser as early as 1977 (Purser, 1977; Purser
et al., 1979). In the early eighties first measurements of platinum, iridium and osmium
in minerals with an ion source modified for Accelerator SIMS were performed at the
University of Toronto. Individual mineral grains were analysed with a caesium beam
with a spot size of several hundred micrometers (Rucklidge et al., 1982). However, it
was in the light of the rapid development of semiconductor technology that the
development of Accelerator SIMS came to life.
In the mid eighties, first test measurements of trace elements in semiconductors were
performed in collaboration between Texas Instruments and the University of Arizona.
The resulting detection limits were about two orders of magnitude better than the best
Chapter 1 Introduction
8
detection limits of SIMS at the time (Ender, 1997a+d). Following this, a new
Accelerator SIMS system was built specifically for the analysis of semiconductors at
the University of Northern Texas, also in collaboration with Texas Instruments
(Anthony et al., 1990; McDaniel et al., 1995).
In the early nineties, the American Semiconductor Industry Association indicated the
need for more sensitive analytical devices to detect impurities in ultra pure materials
and proposed Accelerator SIMS as a possible method (Semiconductor Industry
Association, 1994). In the following years more dedicated systems were built world
wide: A new dedicated ion source for Accelerator SIMS was added to the AMS
facility in Munich (Massonet, 1998) and a new AMS facility with a micro beam
source with a high lateral resolution intended for geological applications was built in
Sydney (Sie et al., 1997a+b).
In more recent years, besides the work presented in this thesis, Accelerator SIMS has
been applied to the measurement of Os isotope ratios in molybdenite (Sie et al., 2002)
and the measurement of tritium depth profiles in walls of fusion reactors (Stan-Sion et
al., 2002).
In 1992 a diploma thesis evaluated the potential of stable trace element analysis in
semiconductors at the PSI/ETH AMS facility in Zurich (Nebiker, 1992). This led to a
doctor thesis (1993–1997) during which the AMS facility was extended with a
commercial SIMS source combined with a specially designed sputter chamber. The
resulting apparatus was tested on trace element analysis in silicon wafers (Ender,
1997a). The detection limits of various trace elements in silicon were the worlds best,
and for some elements, as much as two orders of magnitude better than the best SIMS
detection limits at the time. This apparatus was the one used for the experiments
presented in this thesis.
Apart from these first developments of Accelerator SIMS, work done at the
University of Oxford should be mentioned as well. There, a liquid metal ion source
was used to look at the lateral distribution of 14C in 14C-labeled biological tissue
(Freeman et al., 1994; Jiang et al., 1997). The lateral resolution of the resulting 14C images was ~1 µm.
1.3 Outline of the Thesis
With the PSI/ETH Accelerator SIMS facility up and running and tested on trace
element analysis in silicon wafers, the aim of this thesis is to demonstrate that
Accelerator SIMS can also be used for applications in environmental sciences, and
Chapter 1 Introduction
9
that it has unique features making measurements possible that no other analytical
technique can perform. These unique features, quality, and limits of Accelerator SIMS
measurements will be evaluated by applying the method to two applications out of
environmental sciences and by comparing the results to measurements previously
performed with other analytical techniques. In a final discussion of the potential and
future prospects of Accelerator SIMS, the resulting figures of merit will be compared
with those of other analytical techniques. This leads to the following structure of the
thesis:
In chapter 2 the PSI/ETH Accelerator SIMS facility will be introduced and the
measurements of trace elements in silicon wafers, performed before work towards this
thesis started, will be summarised.
Chapter 3 will give an introduction to parameters influencing an Accelerator SIMS
measurement. In particular, the secondary ion yields of platinum group elements,
gold, and silver (for simplicity abbreviated with PGE) from a silicon dioxide matrix
will be investigated. When confined to analysing negative secondary ions this is of
interest because oxygen is known to enhance the formation of positive secondary ions
in the sputter process, and possibly reduces the negative ion yield as a result. Depth
profiles of implantations of PGE into a silicon dioxide matrix have been measured and
the resulting sensitivities and detection limits will be given.
The procedures used to tune the facility and to measure electrically insulating samples
will also be presented in chapter 3. The PSI/ETH AMS facility in Zurich routinely
analyses radioisotopes only up to a mass of 129 amu. Therefore, when work towards
this thesis commenced, reliable and reproducible procedures to tune the AMS facility
to trace elements of all masses and especially of mass of ~200 amu had to be
developed. Since in most cases environmental samples are electrically non-
conducting, a way to analyse such samples had to be found as well. Technical
improvement of the apparatus and its performance along the way goes without saying.
In chapter 4 the first application will be discussed. It is the in-situ measurement of
iridium concentrations in sedimentary layers around the Cretaceous-Tertiary transition
(KT-boundary). Neutron activation analysis (NAA) revealed that the concentration of
iridium is abnormally high (~56 ng/g) in the transition horizon compared to the
neighbouring sedimentary layers of the Tertiary and the Cretaceous (~0.4 ng/g)
(Pillmore et al., 1987). With Accelerator SIMS it was possible to reproduce the NAA
results, but with a lateral resolution orders of magnitude higher (~100 µm). Presently,
no other technique is capable of performing bulk concentration analysis with such low
trace element concentrations and with such a high lateral resolution. In 1980, Alvarez
Chapter 1 Introduction
10
proposed that the iridium anomaly was caused by a meteorite impact (Alvarez et al.,
1980), but the debate on the origin of the anomalies at the KT-boundary is still on-
going. Due to its capability to perform trace element analysis with a higher lateral
resolution, Accelerator SIMS is therefore a tool capable of analysing this problem in
greater depth.
The second application, which will be presented in chapter 5, is the direct
measurement of natural 10Be/9Be ratios in samples from ferromanganese crusts
(carrier-free 10Be AMS). The instrumental challenge of this application is the
reproducible measurement of isotopic ratios in the 10-10 range in small samples of
only 100 ng in size and to a precision of ~10%. The natural 10Be/9Be ratio as a
function of depth in a ferromanganese crust gives important information on the
growth rate of the crust and is used to date oceanographic events manifested by the
concentrations of other radiogenic isotopes. So far, this information has been gained
with either the direct measurement of the natural 10Be/9Be ratio with a SIMS method
(Belshaw et al., 1995) or with separate measurements of the absolute 10Be and 9Be
concentrations with AMS and ICP-MS respectively. It will be shown that the
Accelerator SIMS results not only reproduce previous measurements very nicely, but
that the method is also more sensitive and measures to a higher precision than the
already existing methods. This means that time scales in ferromanganese crusts can be
determined with a higher precision further into the past.
In chapter 6, the potential of Accelerator SIMS as a method will be compared with
other analytical methods. Especially, a comparison to state-of-the-art SIMS and
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) will be made. These
techniques are widely regarded as the best analytical techniques available today.
Advantages and disadvantages of all methods will be summarised. The final
discussion will also be held in the light of feasible future developments. Especially,
the recent development of small AMS systems (Suter et al., 2000) with high
transmissions of heavy elements such as uranium, plutonium and thorium (> 15% for
Th3+) at terminal voltages of less than 500 kV give reason to assume that Accelerator
SIMS technology can be transferred to small AMS systems as well.
And finally, Chapter 7 will briefly summarise the conclusions drawn from this thesis,
the main achievements of the thesis, and prospects for the future of Accelerator SIMS.
11
Chapter 2 Experimental Setup
In this chapter, the general setup of an Accelerator SIMS experiment together with
some figures of merit of the apparatus will be presented, even though the details of the
experimental setup vary for each of the applications presented in this thesis. But apart
from the different samples, the differences between the applications lie mainly in the
detection system. Special experimental configurations concerning individual
experiments will be introduced in the corresponding chapters.
The tandem accelerator facility in Zurich allows for a wide range of applications in
ion beam analysis. The setup used for Accelerator SIMS is almost identical to the one
used for AMS measurements. The main difference is that Accelerator SIMS analyses
stable isotopes. Contamination coming from the surroundings of the sample in the ion
source is therefore a greater problem. The measurements presented in this work were
all performed with an ion source specially designed to keep such contamination as
low as possible.
2.1 Summary of the Accelerator SIMS Facility
Figure 1 shows the elements of the PSI/ETH tandem accelerator facility that are
relevant for Accelerator SIMS. The ion source was attached to the facility in a way
that the low-energy spectrometer with the highest mass resolution is used
(m/∆m = 330). This guarantees a mass resolution of better than one atomic mass unit
over the entire mass range of the periodic table. The individual components of the
Accelerator SIMS setup are:
• Sputter chamber with the focussed caesium gun
• Secondary ion extraction with small, retractable electrostatic deflector that
bends the beam onto the main beam line of the AMS facility
• Low-energy mass spectrometer consisting of an electrostatic and magnetic
deflector each with a deflection angle of 90° (heavy ion injector)
• 6MV EN Van de Graaff tandem accelerator
• High-energy mass spectrometer with a 15° electrostatic and a 90° magnetic
deflection
Chapter 2 Experimental Setup
12
• Detection chamber with various detector types such as a gas ionisation
detector, Faraday cups, a gas filled magnet or time-of-flight spectrometers
Fig. 1: The PSI/ETH Accelerator SIMS facility. The top half of the picture shows the low-energy part of the facility and the bottom half shows the accelerator with the high-energy mass spectrometer and the detectors.
2.2 The Caesium Source
The first step of an analysis with Accelerator SIMS is to produce negatively charged
ions of the sample material itself. Usually, this is done by bombarding the sample
with a caesium ion beam. The caesium ions produce a cascade of collisions between
the atoms of the sample that cause particles to be ejected out of the sample. This
process is called the sputter process. A certain fraction of the sputtered particles are
negatively charged. It is these secondary ions that are extracted, formed to an ion
beam, and analysed with the rest of the system.
However, when analysing stable isotopes the primary ion beam is a first source of
contamination. Impurities in the caesium are mixed into the sample during sputtering
and will influence the detection limits of the corresponding elements. Therefore,
demands on the purity of the primary ion beam are high.
The caesium gun used for the measurements in this thesis is a Cs431 from Atomika,
Munich. It is an ion source usually used on the Atomika quadrupole SIMS instruments
(Wittmaack, 1992). A cross section through the caesium gun is shown in figure 2.
Caesium vapour from a heated reservoir is ionised due to surface ionisation on a hot
tungsten fritt and an immersion lens accelerates and focuses the ions onto an
Chapter 2 Experimental Setup
13
intermediate point. The beam energy of the caesium gun used for all measurements
was 10 keV. A Wien filter with permanent magnets is used to separate beam
components of different mass. The beam is then sent through one of a series of
selectable apertures mounted on a wheel and with radii between 1 mm and 10 µm.
The objective lens following the aperture focuses the beam onto the sample with a
ratio of about 1:10 so that the beam spot size on the sample - defined as the diameter
of the one sigma range of the current distribution - is between 110 µm and 3 µm.
These focussing optics remained unchanged for all measurements, so, for a given
aperture size the beam spot size will stay roughly the same, whereas the caesium
current can vary due to variations in the performance of the tungsten fritt. Table 1
shows a summary of typical values of Cs current and beam spot size from the
corresponding aperture.
Finally, in order to prevent neutral components in the beam from reaching the sample
the beam is subjected to an electrostatic deflection of 1°. Another two pairs of
deflection plates have been mounted just before the objective lens allowing one to
scan the caesium beam over the sample surface by applying triangular voltage signals
to the plates. The area is scanned with a period of 1 Hz and its size can be
continuously varied from the maximal area possible down to a stationary beam. At a
beam energy of 10 keV the largest area over which the scanning unit is capable of
scanning is approximately 1 mm2.
Cs-Reservoir
Acceleration Lens
Wien Filter
Vacuum Valve
1°-Deflection
Scanning UnitObjective Lens
Wheel withApertures
W-Fritt
Fig. 2: Cross section through the Atomika CS431 caesium gun.
The current of the primary ion beam is stable to within 5 % over a period of 10 hours.
The maximal achievable Cs+ current lies around 600 nA. The Cs current depends
mainly on the size of the chosen aperture.
Chapter 2 Experimental Setup
14
Aperture ∅ Current Beam spot size
1000 µm 455 nA 110 µm
500 µm 208 nA 60 µm
400 µm 133 nA 50 µm
300 µm 98 nA 40 µm
100 µm 13 nA 12 µm
50 µm 3.3 nA 8 µm
10 µm 0.2 nA 2.5 µm
Table 1: Typical values of currents and beam spot sizes of the caesium beam for different aperture sizes at a beam energy of 10 keV.
2.3 The Sputter Chamber
The most important aspect while designing the sputter chamber was the suppression
of contamination originating from the surroundings of the sample during the sputter
process. To achieve this the geometry of the chamber was chosen as open as possible
allowing the secondary particles to travel as far away from the sample as possible
before colliding with the chamber wall and sputtering particles that could return to the
sample as contamination. The design of the chamber together with the nose of the
caesium gun and the secondary ion extraction is shown in figure 3. The wall of the
chamber is at ground potential, so the negative secondary ions have to be accelerated
to ground potential in order to analyse them with the low-energy mass spectrometer
and to inject them into the tandem accelerator. This is done by putting the sample
holder at a potential of –30 kV. The sample holder is mounted on an insulator capable
of holding a voltage difference of 40 kV. The insulator, in its turn, is mounted on an
xy-stage consisting of two in-vacuum stepping motors of Princeton Research
Instruments. The stage allows a movement of 25 mm in steps of 3.3 µm in each
dimension and the position of the sample is monitored with a video camera through a
window in the wall of the vacuum chamber. On its largest magnification the camera
looks at an area of 4 x 6 mm2 on the sample surface. This area is small enough to
accurately position a structured sample. In order to minimise cross contamination,
samples are brought into the chamber one by one through a vacuum lock. They are
mounted in a holder that takes wafer-like objects with a maximal thickness of 4 mm
Chapter 2 Experimental Setup
15
and side lengths of 28 mm. The design of the holder is such that the surfaces of
samples with different thickness are held at the same well-defined place.
Since the sputter processes of not-extracted ions in the vicinity of the sample and the
chamber walls are the main source of contamination, the sample holder, the extraction
lens, and the caesium gun nose are coated with a 20 µm thick layer of pure gold
(99.99%). Gold was chosen because of its availability in a very pure form and because
the coating of steel is technically easy. In addition, the geometry of the lenses are such
that as little area as possible is visible from the sample and therefore exposed to the
unwanted bombardment of secondary ions.
Fig. 3: Design of the Accelerator SIMS sputter chamber.
To keep the effects of residual gas in the chamber as low as possible the chamber is
built with UHV-technology. It is pumped with a 450 l/s turbo pump which achieves a
base vacuum of 3⋅10-10 mbar. During measurement, the valve to the main beam line is
opened and together with the changing of samples and the operation of the caesium
gun the pressure in the chamber rises to 10-8 mbar. The caesium gun itself is pumped
with a 50 l/s getter pump.
As can be seen in figure 3 the primary caesium beam has an angle of incidence of 30°
relative to the normal of the sample surface. Symmetrically, the secondary ions are
extracted with a three-step extraction lens also at an angle of 30°. This geometry was
convenient when attaching the ion source to the rest of the tandem facility, and since
Chapter 2 Experimental Setup
16
the focal point of the caesium gun is 20 mm in front of the end of its nose, the
geometry also allows for an optimal exploit of the physical properties of the sputter
process. The angle of incidence of 30° also favours a higher depth resolution when
measuring depth profiles. A detailed discussion of the ion optics of the sputter
chamber can be found in (Ender, 1997a).
During operation the entire caesium gun with all its control electronics is put on a
potential of –30.8 kV. The nose of the caesium gun is therefore at a potential of
−0.8 kV relative to the sample stage and the Cs beam is decelerated to an energy of
9.2 keV before hitting the sample. This prevents negative secondary ions from being
accelerated back onto the nose of the caesium gun and sputtering contaminating
particles. In addition the first electrode of the extraction lens is typically set to a
potential of –27.2 kV. The large potential difference to the sample ensures that as
many negative ions as possible are extracted into the beam line. The second extraction
electrode is typically set to a potential of –16.6 kV that, together with a quadrupole
lens and two electrostatic steerer units, guides the secondary ion beam to the object
slits of the small electrostatic deflector.
2.4 The Low-Energy Mass Spectrometer
The small, spherical, electrostatic deflector with a trajectory radius of 12 cm bends the
beam by 90° onto the beam line of the so-called heavy ion injector shown in figure 4.
It creates an image of the object slits in the object plane of the second, larger
electrostatic deflection unit. This unit also deflects the beam by an angle of 90°, but
with a trajectory radius of 75 cm. Its stigmatic image lies in the objet plane of the 90°,
stigmatic injection magnet with a radius of 60 cm. The mass resolution of the
resulting mass spectrometer is m/∆m = 330 and high enough to separate every mass in
the periodic system (Synal et al., 1991).
Faraday cups can be inserted into the beam line at the focal points of the larger
electrostatic and magnetic deflection units. Even though the beam intensities are
usually three orders of magnitude smaller than in AMS measurements, the secondary
ion currents of matrix elements are usually strong enough to be measured with in a
Faraday cup. However, for the measurement of a conventional SIMS spectrum, weak
ion currents can be measured by amplifying the secondary electrons produced by the
ion beam in a cup with a channel plate electron multiplier. Using such a secondary
electron multiplier the low-energy magnet can be tuned to every mass in the entire
mass spectrum (Ender, 1997a; Maden, 1998).
Chapter 2 Experimental Setup
17
Fig. 4: Accelerator SIMS ion source with the heavy ion injector. The first electrostatic deflection unit can be retracted out of the main beam line of the facility.
2.5 The Accelerator and High-Energy Mass Spectrometer
After passing the heavy ion injector the negative ions are accelerated towards the
terminal of the 6 MV EN tandem accelerator. If not mentioned otherwise, all
measurements presented here were performed with a terminal voltage of 5.0 MV. At
the terminal the ions pass through a so-called stripper. Due to collisions with the
atoms of the stripper medium the ions lose a few electrons and become positively
charged. In addition, molecules in charge states higher than 2+ become unstable and
disintegrate. The strippers used were either a carbon foil, 3 µg/cm2 thick, or
differentially pumped argon gas (Niklaus, 1993; Niklaus et al., 1994). The positively
charged ions are then accelerated away from the terminal back to ground potential
giving them a total energy E = UT⋅(mHE/mLE + q)⋅e where UT is the terminal voltage,
mLE the mass of the ion on the low-energy side of the accelerator, mHE the mass of the
ion on the high-energy side after a possible molecular split-up, and q the charge state
of the ion after passing through the stripper.
At the exit of the tandem accelerator a second, retractable channel plate electron
multiplier has been installed. It is identical to the one on the low-energy side of the
tandem and its purpose is to aid the tuning of the low-energy magnet during
measurements. The electron multiplier was installed during work towards this thesis
Chapter 2 Experimental Setup
18
and has made tuning of the facility for analysis of trace elements in the entire mass
spectrum possible. One could be tempted to use it to improve the resolution of the
low-energy mass spectrometer when recording conventional SIMS spectra. Due to its
greater distance from the low-energy magnet the mass resolution is indeed improved.
However, there has to be a voltage applied to the terminal of the accelerator in order
to focus the ion beam onto the Faraday cup. Since molecules are split into their
compounds at the terminal and produce different amounts of secondary particles in
the accelerator, the composition of the ion beams of two neighbouring masses can be
different, and the ratio of the measured currents is not equal to the ratio of the
corresponding particle intensities.
Fig. 5: High-energy side of the PSI/ETH tandem facility.
After the tandem accelerator, the ion beam is subjected to an electrostatic deflection
of 15° with a radius of 5.8 m as seen in figure 5. The resolution of this E/q-filter is 1-
2 % depending on the width of the aperture following the deflector. A magnetic
deflection of 90° with a radius of 1.1 m performs a final p/q-analysis before the ions
reach the detector. For a given charge state q the width of the detector window used in
a typical experiment gives a resolution ∆p/p of 0.4 %.
Apart from the retractable Faraday cup after the accelerator, a retractable cup can be
inserted into the beam line after the electrostatic deflector and another cup is installed
after the magnetic analyser.
2.6 Particle Identification
For bulk concentration analysis of a trace element only two kinds of detectors are
necessary. The current of the matrix elements can be measured in a Faraday cup. With
the existing setup currents of a few 10-12 A can be measured (1 pA in charge state 1+
corresponds to a particle counting rate of 6 MHz). Particle rates of less than 2 kHz are
Chapter 2 Experimental Setup
19
measured with a gas ionisation detector. For the experiments presented here, there
was no need to close the gap between the two ranges since the concentrations of the
measured trace elements are very low.
+500 V
+300 V
Cathode BoxEtot
Entrance Window(Mylar)
Ion Beam
∆E-Anode ER-Anode
Frisch Grid
∆E ER
A
A
A
Fig. 6: Design of the gas ionisation detector used for most of the experiments. The first electrode has a length of 50 mm and the second one is 190 mm long. The total energy signal of the detected particles is not read into the data acquisition system.
However, all particles with the same m/q ratio will pass the high-energy mass
spectrometer on the same trajectory. This means that the detector has to be able to
resolve interferences from the trace isotope of interest. These interferences can be
isobars in the same charge state and molecular fragments in different charge states as
the trace isotope. The gas ionisation detector used is filled with a mixture out of argon
(90%) and methane (10%). The entrance window is a 100 µg/cm2 thick mylar foil.
The complete design of the detector is shown in figure 6. Ions arriving at the detector
penetrate the mylar entrance window and are stopped in the gas of the detector. The
gas is ionised along the track of the ions creating charged particles that drift along an
electric field perpendicular to the trajectory of the ions until they are collected on its
electrodes. The first electrode is 50 mm and second one is 190 mm long. Charge
sensitive amplifiers then send the signal on to the data acquisition system.
The electronic stopping power, dE/dx, of the incoming ion has a dependency on the
nuclear charge of the projectile, Z, which can be written as the square of the mean
charge of the projectile, <q(Z)>, times the stopping power of a proton at the same
velocity, v. (Betz, 1972 and 1983)
P
2 )v(dx
dE)Z(q)v(
dx
dE
⋅><= (2.1)
Chapter 2 Experimental Setup
20
The amount of detector gas ionised depends on the amount of energy lost by the
projectile along a given path length. The charge collected on the electrodes is then
proportional to the energy lost by the ion in form of ionisation along the
corresponding path lengths.
∫
∫+
=
=∆
21
1
1
ll
l
R
l
0
dxdx
dEE
dxdx
dEE
(2.2)
where ∆E is the energy signal coming from the first electrode and ER the residual
energy signal from the second electrode. Due to the dependency of the stopping
power on the nuclear charge, ∆E and ER are also dependent on the nuclear charge of
the projectile and a separation of isobaric interferences is possible. The separation of
molecular fragments in lower charge states is also very efficient with this detection
method due to the lower total energy of these ions. Figure 7 demonstrates the
separation of isobars on the example of 54Fe and 54Cr in a histogram of ∆E versus ER.
Gates can be set around a peak in order to accept only the counts of the desired
isotope.
Fig. 7: Histogram of the energy signals of 40 MeV 54Fe and 54Cr from the gas ionisation detector. The peaks are clearly separated.
Further advantages of gas ionisation counters are that they have virtually no
background counting rate and a response probability of 1. In addition, a certain
amount of energy that is lost by the projectile via other channels than electronic
Chapter 2 Experimental Setup
21
stopping is also converted into ionisation of the counter gas. In particular, a large
fraction of the projectile energy is transferred to recoiling gas atoms at energies below
0.1 MeV/amu. The energy lost by the projectile in such scattering processes does not
ionise the counter gas directly. However, the recoiling gas atom ionises the detector
gas during its stopping process and a certain amount of its energy is converted into
ionisation energy. When analysing low-energetic beams (< 2 MeV) and heavy trace
elements, this property gives gas ionisation detectors a better energy resolution and
smaller pulse-height defects in comparison to other detector types, such as
semiconductor detectors.
2.7 Data Acquisition
During the construction of the Accelerator SIMS setup the data acquisition system of
the AMS facility was modified to accommodate for the needs of Accelerator SIMS
(Synal et al., 1997; Ender et al., 1997d). The main difference is that for every trigger
event not only the amplitude of the ∆E and ER signals are read by the acquisition
system, but also the momentary values of two triangular voltage signals that are
proportional to the lateral deflection of the Cs beam by its scanning unit. Therefore,
not only information on the energy of the particles is acquired, but also information on
where on the sample surface the particle originated from.
In figure 8 one can see the most important spectra used during a measurement. These
are the multichannel spectra of ∆E and ER and the two 2-dimensional histograms in
which on one hand the intensity distribution of ∆E and ER are plotted and on the other
hand the distribution of the trace isotope on the sample surface. The spectra are
recorded in a way that at first all events are plotted in the ∆E-spectrum. In the ER-
spectrum, however, only those events will be plotted that lie in a so-called pre-gate in
the ∆E-spectrum. And further, only those events will be plotted in the 2-dimensional
∆E/ER-histogram which lie in a second pre-gate in the ER-spectrum. Last of all, only
those events lying in a gated area of the ∆E/ER-plane will be taken into the histogram
with the xy-distribution. This procedure reduces the number of interfering particles
step by step.
During a measurement the number of counts in the gates is measured over an
adjustable length of time, the so-called cycle time, before they are written into the
database. Depending on the experiment the cycle time is chosen between 10 and 60
seconds. After a cycle the measurement is paused for a couple of seconds to write the
acquired data to the database and to start a new cycle. One parameter written into the
Chapter 2 Experimental Setup
22
database is the effective measuring time of the cycle. This takes instabilities of the
terminal voltage, dead time of the detector, and the low-energy bouncing system into
account. The real counting rate in the detector is therefore the number of acquired
counts per effective measuring time.
Fig. 8: To illustrate the data acquisition system the two multi-channel spectra of the
∆E- and ER-signals are shown (top row) together with the two 2-dimensional histograms,
the ∆E/ER-spectrum and the lateral distribution of the events on the sample surface. Only
events lying within the rectangular gate of the ∆E/ER-spectrum are plotted in the xy-
histogram.
There is also the possibility to gate an area of the xy-plane. This is useful when
measuring depth profiles, which are recorded by regarding the counting rate of the
detector as a function of time and then normalising to the depth of the sputtered crater.
For all depth profiles presented in this work the central 15% of the xy-plane was
gated. This fraction was chosen based on the fraction of a sputter area of
500 x 500 µm2 that is left when the size of the area is reduced by two beam diameters
(two times ~110 µm) in both dimensions. Of course, this reduces the analysed volume
of the sample by a factor of 7 but also increases the depth resolution, because the
sensitive area is confined to the central, plane part of the sputter crater and
concentration differences on the crater wall are ignored.
2.8 Figures of Merit of Accelerator SIMS
Before the work towards this thesis began, the Accelerator SIMS setup had been
tested on the detection of trace elements in silicon wafers. Bulk detection limits of
various trace elements, depth resolution, and lateral resolution had been determined
Chapter 2 Experimental Setup
23
and found to be the best in the world. This section will give a short summary of the
most important results. It should be kept in mind that the apparatus has been improved
over the last four years and with it the detection limits have been reduced. The values
presented here are therefore to be regarded as upper limits. It is estimated that the
figures have been improved by more than one order of magnitude mainly due to
higher transmission through the mass spectrometers and the tandem. This was mainly
achieved by using a different ion optical configuration to inject the ion beam into the
tandem accelerator. In addition, the designs of the extraction lens and the small
electrostatic deflection unit were improved to achieve better stability of the secondary
ion currents.
For the determination of the detection limits of the elements B, Al, P, Fe, Ni, Cu, As
and Sb it was possible to use silicon wafers with a known concentration of the trace
elements as standards. The measured counting rates were then compared to the
counting rate coming off a corresponding blank and the ratio of the two give the
detection limit. This procedure has also been described in (Maden, 1998). Table 2
summarises the detection limits.
Trace Element Detection limit
[at./Si]
Detection limit
[cm-3]
B 1⋅10-10 7⋅1012
Al 7⋅10-11 4⋅1012
P 3⋅10-10 2⋅1013
Fe 1⋅10-9 1⋅1014
Cu 1⋅10-9 1⋅1014
As 9⋅10-12 5⋅1011
Sb 2⋅10-10 1⋅1013
Table 2: Bulk detection limits of trace elements in a silicon matrix of the Zurich Accelerator SIMS facility.
The capability of the facility with respect to depth profiling is shown in figure 9. It
shows an example of a depth profile of a 300 nm thick Al doped layer on a pure
silicon substrate. The top picture is a light microscope image of the sputter crater
together with a profile of the crater measured with a Dectac profilometer (middle).
The bottom picture shows the intensity of the Al signal as a function of depth into the
sample. The layer thickness in which the Al intensity drops from 84% down to 16% is
23 nm, thus giving the depth resolution.
Chapter 2 Experimental Setup
24
Position [mm]
Dep
t h [m m
]
0 100 200 300 400 500
0
1
2
Position [mm]
Dep
t h [m m
]
0 100 200 300 400 500
0
1
2
Fig. 9: Light microscope picture of a sputter crater (top) and the corresponding profile measured with a Dectac profilometer (middle). The roughness of the bottom of the crater is 5 nm. The bottom picture shows the relative intensity of sputtered Al as a function of depth into the sample.
A nice demonstration of the imaging properties of the facility is shown in figure 10.
An area (500 µm x 500 µm) of an integrated circuit chip (shown as an optical
micrograph in the right hand picture) was analysed with respect to its lateral
phosphorous concentration. The resulting image is shown in the top left hand image.
The P concentration is grey scale coded with white being regions with a high
P concentration and black the regions with low concentration. The bottom left hand
picture shows the concentration distribution along a line scan on the same area
(dashed line in the top left hand picture). The lateral resolution of the image is 4 µm.
Chapter 2 Experimental Setup
25
Fig. 10: Distribution of phosphorous on an area of an integrated circuit chip
(500 µm x500 µm). The P concentration varies between 1014 and 1017 at./cm3 and the
lateral resolution of the image is 4 µm.
1018
1017
1016
1015
1014
1013
0 100 200 300 400
PC
once
ntra
tion
[at/
cm3 ]
Position (mm)
1018
1017
1016
1015
1014
1013
0 100 200 300 400
PC
once
ntra
tion
[at/
cm3 ]
Position (mm)
27
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
Accelerator SIMS measurements are subjected to the same physical constraints as
conventional SIMS measurements are. Both methods primarily rely on an optimal
yield of secondary ions from a sample. Owing to the fact that positive ions are not
accelerated to the terminal of the tandem accelerator, Accelerator SIMS is
additionally confined to the analysis of negative secondary ions. In addition, for in-
situ analysis no chemical preparation of the sample precedes the measurement. In
cases, this can make it more difficult to find a suitable reference material to use as a
standard sample. This becomes more apparent when noting that the size of a sample is
a lot larger than the beam spot of the Cs beam. If the distributions of the matrix
elements are homogeneous throughout the sample an adequate standard can usually be
found. In a sample with a non-uniform matrix composition, however, the secondary
ion yield of a given trace element will vary with the lateral position of the Cs beam
even if the concentration of the element is homogeneous throughout the sample.
For this reason it is vital to study the sputter properties of a sample matrix when
assessing the feasibility of applying Accelerator SIMS to an application in a field of
research. This chapter will demonstrate the importance of this issue on the example of
the analysis of platinum group elements, gold and silver, which for simplicity will be
referred to as PGE in the following, in silicon and silicon dioxide matrices, a topic of
great interest for the application of Accelerator SIMS in the field of geology. The
results of the discussion will then be applied to a practical problem in chapter 4 where
Accelerator SIMS measurements of PGE in sedimentary layers at the Cretaceous-
Tertiary transition (KT-boundary) are presented.
3.1 Formalisms
First, however, a few terms should be introduced. This section makes no claim for
completeness and will only introduce terms necessary to understand the procedure of
an Accelerator SIMS analysis and the following discussion. A more complete treatise
can be found in (Benninghoven et al., 1989).
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
28
3.1.1 Sputter yields
The physical details of the sputter process have not been understood in detail yet. It is
clear that the incident primary ion produces a cascade of collisions between the atoms
of the sample resulting in particles being ejected out of the sample. Several models
and simulations of the sputter process exist (Sigmund et al., 1974; Ziegler et al.,
1985), but often the direct measurement remains the securest source of information.
For the purpose of Accelerator SIMS, however, it is not necessary to know the details
of the sputter process. The formalisms used regard the sputter process as a black box.
All one needs to know is how many target atoms are sputtered on average per
incoming primary ion and the probability of formation of a secondary ion.
The amount of sample material used in a measurement is determined by the total
sputtering yield, Ytot. It is defined as the average number of sample atoms ejected out
of the sample per incoming primary ion. For example, the total sputter yield of pure
silicon resulting from 10 keV Cs+ ions incident at 30° from the sample normal has
been measured to be Ytot = 2.4 (sputtered Si atoms per Cs ion) (Ender, 1997a).
Usually the composition of the sample matrix is not monatomic, but consists of
several elements (A, B, C, …) with a molecular stoichiometry (AiBjCl) defining the
relative concentrations of the matrix elements. In this case, the total sputter yield is
given in units of number of sputtered molecules AiBjCl per incident primary ion.
The types of secondary particles sputtered from the sample are plentiful. Apart from
secondary electrons, they can be either single atoms or molecular particles of all
possible compositions and in different charge states. If Yq(M) is the sputter yield of
molecule M in charge state q, i.e. the average number of molecules in charge state q
sputtered per incoming primary ion, then the total sputter yield of a specific
element A, Ytot(A), is given by summing over all charge states q and molecule types
Ml = AiBjCk
∑∑∑∑ ==q k,j,i
kjiq
q ll
qtot )CBA(iY)M(iY)A(Y (3.1)
The total sputter yield of the sample is the sum of the total specific sputter yields of all
the elements in the matrix.
γβα)()()(
)()()(CYBYAY
CYBYAYY tottottottottottottot ===++= (3.2)
where α,β and γ are such that the matrix stoichiometry can be written as AαBβCγ with
α + β + γ = 1.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
29
The same formalism can be used for elements consisting of more than one isotope. In
this case the sum in equation 3.1 also runs over the different isotopes of element A as
well. Thus
∑ ==N
N
NtotN
tottotAr
AYAYAY
)(
)()()( (3.3)
where AN symbolises an individual isotope of element A, and r(AN) is the
corresponding isotopic abundance.
Here it has been assumed that the total sputter yields of all elements in the matrix are
linearly correlated to their relative concentrations, so-called stoichiometric sputtering.
Therefore there is no enrichment of an element or an isotope during the sputter
process. By general reasons every equilibrium sputter process is stoichiometric. This
also means that the fractional concentration, c(A), of element A can also be written as
tot
tot
tot Y
)A(Y
n
)A(n)A(c == (3.4)
With n(A) being the volume density of element A in the sample [atoms/cm3] and ntot
the total number of target atoms per cm3.
3.1.2 Secondary ion currents
The total sputter yield of a sample matrix is useful to estimate the total amount of
sample material required to perform an analysis for a certain time interval. The
quantities measured during a measurement, however, are currents of secondary ions.
The probability of the creation of a secondary ion in the sputter process therefore
influences the characteristics of a measurement. Occasionally it is of advantage to
analyse a sputtered molecule P instead of the atomic ion A itself, because not all
elements form stable negative ions and also only a few per cent of the sputtered
particles leave the sample as ions. Over 90% of the particles are sputtered as neutrals.
Therefore, if A is the element of interest for an analysis and Lk
Mj
N CBAP = is a
particle containing isotope N of element A, then one can define the probability of
ionisation, αq(P), of isotope AN contained in particle P into a given charge state q as
follows:
)(
)()(
AY
PYP
tot
qq =α (3.5)
After extracting ions of charge state q to a beam and passing them through a mass
spectrometer the resulting current, Iq(P), can be described as
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
30
)()()()()( PPTqPYAcIPI qqtotp
q εα ⋅⋅⋅⋅⋅⋅= (3.6)
where Ip is the current of the primary beam, c(A) the fractional concentration of A,
Tq(P) the transmission probability of P through the mass spectrometer, and ε(P) the
efficiency of the detector.
Equation 3.6 is enough to calculate the secondary ion current for SIMS, that is, for an
ion current that has passed through a single mass spectrometer. For Accelerator SIMS
one could at most claim validity for atomic ions extracted from the sample. This is
because the ion beam is additionally passed through the tandem accelerator and the
high-energy spectrometer breaking up molecules in the stripper. Hence for
Accelerator SIMS equation 3.6 becomes
)()()()()()()( NNqHE
NqELEtotp
Nq AATqAPTPYAcIAI εβα ⋅⋅⋅⋅⋅⋅⋅⋅= −− (3.7)
where the transmission Tq(P) has been substituted with the product of the
transmission of the negatively charged particle P through the low-energy
spectrometer, )(PTLE− , the probability of the isotope N of element A populating
charge state q after having passed the stripper at energy E, )( NqE Aβ , and the
transmission through the high-energy spectrometer, )( NqHE AT .
This distinction between SIMS and Accelerator SIMS will not be made in the
following. Since the secondary electrons follow the same trajectory as singly negative
ions until they reach the first magnetic deflection, the first place in the beam line that
a pure ion current can be measured is after the low-energy magnet. For the PSI/ETH
Accelerator SIMS facility it is therefore advantageous to make the following
definitions. The product )()()( NNqHE
NqE AATA εβ ⋅⋅ is described as the transmission,
)( NqE AT , of isotope AN in charge state q and at energy E. Likewise the yield, )(PN M
− ,
of a specific secondary ion P from a sample is understood to be the product
)()( PTP LE−− ⋅α , that is, the probability of an isotope AN reaching the low-energy side
of the accelerator as a negative ion P per sputtered atom of element A. As will be
shown in section 3.1.4 the yield depends on the type of matrix M the ion is sputtered
from. And finally, the product of the yield and the transmission is called the overall
useful yield, )(,,
NEqMP AU . It describes the probability of detecting an isotope AN in
charge state q per sputtered atom A from the sample while analysing at energy E.
)()()()()()()()(, NNqHE
NqELE
NqEM
NEqM AATAPTPATPNAU εβα ⋅⋅⋅⋅=⋅= −−− (3.8)
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
31
3.1.3 Quantification formalisms
There are different formalisms used to make a quantification of the concentration of a
trace element in a measurement. Two of them shall be presented briefly.
The first one uses so-called Relative Sensitivity Factors (RSF). This formalism relies
on the availability of a standard sample with a known concentration of the trace
element in order to determine the value of the RSF. Further, exactly identical
analytical conditions are assumed for the measurement of the standard sample and for
that of the unknown sample. And last of all, the concentrations of the trace element in
the standard and the sample have to be so low that its secondary ion current is
proportional to the absolute concentration of the trace element. This is the case, if the
concentration is below ~1%. If these conditions are fulfilled, it is sufficient to measure
the secondary ion current of one of the isotopes of trace element S and the ion current
of one of isotopes of the matrix element A of the sample matrix M ( M = AiBjCk )in
order to obtain the concentration of the trace element in the sample:
)()(
)()(
'NA
MKq
Nq
SRSFAI
SISn
K
⋅= (3.9)
Here n(S) is the concentration [atoms/cm3] of the trace element S in the sample. Iq(SN)
is the measured secondary ion current of isotope N of the trace element S in charge
state q, and Iq’(AK) is the measured ion current of isotope K of element A in charge
state q’. The relative sensitivity factor, )( NAM SRSF
K
, is also calculated from equation
3.9, but with the values gained from the measurement of the standard sample. It is
clear from equation 3.9 that the use of a given RSF is limited to trace element analysis
performed under exactly identical experimental conditions. In particular, this includes
the isotope SN of the analysed trace element, the matrix isotope AK, and the
stoichiometry AiBjCk of the sample as well.
Equation 3.9 is strikingly simple and concentration calibration for a measurement
becomes an easy task. However, it is also clear that the RSF has the dimensions of a
number density [atoms/cm3]. This can be irritating in as much that the absolute value
of the RSF is inversely proportional to the relative sensitivity of Accelerator SIMS
analysis of different trace elements in a given matrix.
To overcome this problem the introduction of Scaled Sensitivity Factors (SSF) and
Scaled Sensitivity Ratios (SSR) has been proposed (Wittmaack, 1995). Starting from
equation 3.7 one can define a Scaled Sensitivity Factor for a specific matrix M and
element S as follows
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
32
tot
totNNq
EMN
p
NqS
M n
YSSTPN
qSnSrI
SISSF
⋅⋅⋅=
⋅⋅⋅≡
− )()()(
)()(
)( ε (3.10)
As can be seen from its units [(counts/s)/(nA⋅atoms/cm3)] the Scaled Sensitivity
Factor describes the secondary ion current of an element S to be expected from
matrix M per unit current of the primary beam and per unit concentration of the trace
element S. Likewise a Scaled Sensitivity Factor, AMSSF , can be defined for a matrix
element A as well. Taking the ratio of the two SSF defines the so-called Scaled
Sensitivity Ratio, SMASSR ,
)()()(
)()()(
)()()(
)()()('', SnSrAI
AnArSI
AATAN
SSTPN
SSF
SSFSSR
NKq
KNq
KKqEM
NNqEM
AM
SMS
MA ⋅⋅⋅⋅=
⋅⋅⋅⋅
=≡ −
−
εε
(3.11)
Just like for Relative Sensitivity Factors the Scaled Sensitivity Ratio has to be
measured with the aid of a standard sample. Concentration calibration is now
performed based on the following equation
SMA
NKq
KNq
SSRSrAI
ArSI
An
Sn
,'
1
)()(
)()(
)(
)( ⋅⋅⋅= (3.12)
Comparing equation 3.12 with equation 3.9, it becomes clear that the SSR is basically
an RSF with the dependence on the concentration of the matrix element A decoupled
from it. In addition, the dependence on the isotopic ratio has been removed making
the SSR a more instructive measure for planning an experiment, since it is the ratio of
the overall useful yields of the two elements under consideration. Its absolute value is
therefore directly proportional to the sensitivity of SIMS analyses of a trace element S
in a given matrix M when using the element A as a reference. The currents can also be
estimated with the Scaled Sensitivity Factors.
However, since a change in the matrix composition would cause an unpredictable
change in the secondary ion yields, the SSF, and with them the SSR, would change
imposing the same restriction as on the RSF: the Scaled Sensitivity Ratio is only valid
for trace element analyses performed under exactly identical experimental conditions
as the ones during the analysis of the corresponding standard sample, in particular, the
same matrix composition.
3.1.4 Parameters influencing secondary ion yields
Both formalisms presented above require measuring a standard sample with a known
concentration of the trace element before a concentration calibration can be made for
an unknown sample. To date, these are the most reliable calibration methods. This is
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
33
due to the lack of a theoretical model capable of predicting the secondary ion yields of
trace elements from a matrix. All that is available are semi-empirical models based on
collections of data on secondary ion yields form different matrices. Usually, a model
is only valid within a group of matrices with similar physical properties. The different
models use different parameters to predict the secondary ion yield. In the following,
the secondary ion yield data gained from measurements on metals, metal-silicides and
semiconductors with Cs+ and O+ primary beams will be discussed as an example.
For a given set of primary ion beam parameters the properties influencing the
secondary ion yield have to be related to the trace element on one hand and the
sample matrix on the other. This is clear, because the secondary ion yields vary for
different trace elements sputtered from the same matrix and for a given trace element
sputtered from different matrices (Deline et al., 1978a). Specifically, considering the
case of different trace elements in a matrix of the kind mentioned above, it has been
found that the secondary ion yield forms a Saha relation with the electron affinity ΩT
for negative secondary ions and with the ionisation potential ΦT for positive
secondary ions. That is
T
T
eS
eSΦ−−
Ω−+
∝
∝
)(
)(
αα
(3.13)
In the second case, where a given trace element is sputtered from different
matrices M, it was found that the linear sputter rate Sl,M [nm/s], i.e. the speed at which
the sample surface is eroded, influences the secondary ion yield. The reason being that
the concentration of the element of the primary beam at the surface of a sample
influences the properties of a matrix with respect to the sputter process. The
concentration of the primary species, for example n(Cs), at the sample surface is
proportional to the inverse of the linear sputter rate: n(Cs) ∼ 1/Sl,M. The overall
relation between the secondary ion yield and the linear sputter rate for some matrices
has been found to be (Deline et al.,1978b)
[ ] +
+
∝
∝+ S
S
x
x
Ml
OnS
S )(1
)(,
α (3.14)
or
[ ] −
−
∝
∝− S
S
x
x
Ml
CsnS
S )(1
)(,
α (3.15)
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
34
The primary species O+ and Cs+ were chosen as examples, because an O+ beam
favours the yield of positive secondary ions and a Cs+ beam favours negatively
charged secondary ions. The exponents xs± vary from trace element to trace element
and it is not yet clear on which parameters they depend. Table 3 gives their values for
some trace elements.
Fig. 11: Useful Si- yield versus the reciprocal of the linear sputtering rate Sl,M for various metal silicides measured on an AEI-IM20 ion microprobe (Deline et al., 1978b).
Relations 3.1.14 and 3.1.15 also hold for a given matrix element sputtered from
different binary compound matrices. This has been demonstrated on the yield of
negative silicon ions from different metal silicides as can be seen in figure 11.
Element xs+ xs
-
B 2.8 ± 0.1 2.1 ± 0.1
C 2.0 ± 0.1
P 2.8 ± 0.3 2.4 ± 0.2
As 3.3 ± 0.1 2.6 ± 0.1
Sb 2.9 ± 0.3 3.4 ± 0.2
Table 3: Exponents of the reciprocal of the linear sputtering rate, Sl,M, for positive and negative ionisation probabilities with an O+ or a Cs+ primary beam respectively (Deline et al., 1978b)
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
35
3.2 Negative Secondary Ions from a SiO2 Matrix with a Cs+ Primary Beam
By the nature of the method, Accelerator SIMS is limited to analysing negative
secondary ions. This can be a problem if a sample matrix contains oxygen as one of
its main compounds, for example SiO2. As mentioned above, the presence of oxygen
favours the yield of positive secondary ions. When experimentally confined to using a
negative ion enhancing caesium beam the question arises as to how the coexistence of
Cs and O will influence the yield of negative secondary ions from a matrix such as
SiO2. This question is an important issue when wanting to apply Accelerator SIMS to
geological samples, the matrices of which often contain oxides. Traditionally, SIMS
analysis of geological samples is performed with an oxygen primary beam while
extracting positive secondary ions. In the following, an experiment performed at the
PSI/ETH Accelerator SIMS facility with the goal to measure the yield of negative
secondary ions from a SiO2 matrix will be presented. The trace elements used for this
experiment were the platinum group elements, gold, and silver (PGE), because of
their potential for environmental applications.
3.2.1 Samples and implantation technique
As mentioned in chapter 2, the design of the sputter chamber requires samples of
wafer-like shape. Therefore, the idea was to take a structured wafer containing a SiO2
layer and a Si substrate, and to implant a given PGE at different energies but on the
same place on the wafer. This way a concentration maximum of the PGE is produced
both in the SiO2 layer and the substrate. The ion yield of the PGE in SiO2 relative to
the yield from a Si matrix can then be determined by measuring a depth profile of the
implantation with Accelerator SIMS.
The wafer chosen for the experiment was produced with the bond-and-etch-back
technique, but for simplicity it will be referred to as a SIMOX wafer (separation by
implantation of oxygen). Its surface consists of a 200 nm thick layer of silicon.
Beneath it is a 100 nm thick SiO2 layer, which in its turn is on a silicon substrate.
Figure 12 shows a cross section through the wafer. An important aspect to the choice
of the wafer is that the silicon surface layer acts as an electrically conducting
protection on top of the insulating silicon dioxide layer. Charging of the sample from
the primary beam is therefore prevented.
The implantations were made by inserting a pure metal PGE cathode (with the
exception of osmium where an OsCl3 cathode was used) into the high-current source
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
36
of the AMS facility (see also figure 29 in chapter 5). For each element the isotope
with the highest abundance was selected with the low-energy mass spectrometer. The
implantation targets were placed at the focal point of the beam behind the high-energy
electrostatic deflector. Thus a charge state of the positive ions produced at the stripper
can be selected and together with the terminal voltage of the tandem accelerator the
final implantation energy is known to better than 1%. The PGE beam was scanned
over an aperture just in front of the implantation target. The size of this aperture
defines the size of the implantation area on the target surface. It was set to 4 x 4 mm
and left unchanged for all implantations. Measuring the mean current of the scanned
ion beam in a Faraday cup behind the aperture before and after exposing the target to
the ion beam, together with the measured time of exposure, the implantation fluence
can be determined to within 2%.
The isotopes 103Rh, 106Pd, 107Ag, 192Os, 193Ir, 195Pt, and 197Au of the PGE were
selected for the experiment. Implantation energies were 0.60 MeV and 2.00 MeV for
the isotopes 103Rh, 106Pd, and 107Ag and the isotopes 192Os, 193Ir, 195Pt, and 197Au were
implanted at 0.95 MeV and 3.50 MeV.
200 nm
100 nm
Si
Si-Substrate
SiO2
15o
PGE+
Fig. 12: Layer structure of a SIMOX wafer with the direction of incidence of the PGE indicated
The energies were chosen based on TRIM simulations. The only criterion was to
achieve a concentration maximum in the centre of the SiO2 layer, and separate from it,
a concentration maximum in the substrate. Figure 13 shows such a simulation of the
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
37
depth dependence of the concentration of 195Pt in the SIMOX wafer. It is
representative for all the implanted isotopes and shows that not all of the projectiles of
the lower implantation energy are stopped in the SiO2 layer. Average ranges and
longitudinal straggling (1σ) of the different isotopes in the SIMOX wafer are
summarised in table 4. The differences in the ranges for isotopes of nearly the same
mass lie within the error of the simulation. Therefore, in table 4 only a distinction
between the light PGE (Pd, Rh, Ag) and the heavy PGE (Os, Ir, Pt, Au) has been
made.
Fig. 13: TRIM simulation of the concentration of 195Pt as a function of depth in the SIMOX wafer.
Isotopes Range at low
energy
Straggling at
low energy
Range at high
energy
Straggling at
high energy
103Rh, 106Pd, 107Ag
245 nm 64 nm 822 nm 160 nm
192Os, 193Ir, 195Pt, 197Au
244 nm 52 nm 780 nm 150 nm
Table 4: Ranges and longitudinal straggling (1σ) taken from the TRIM simulation for
the implanted isotopes.
1E+12
1E+13
1E+14
0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
Depth (nm)
Ato
mic
Con
cent
rati
on (
a.u.
)
SiO2 Layer
3.5 MeV-Implantation950 keV-Implantation
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
38
The aim was to implant doses of 1⋅1014 atoms/cm2 for all isotopes at each energy. Due
to variations in the ion current during exposure of the wafer, the implanted doses -
with the exception of Pd - were up to 10% away from this value. The small current
extracted from the Pd cathode only allowed implantation doses of 1⋅1013 atoms/cm2.
The implantations at the higher energy were performed first as not to disturb the
shallower lying profile during implantation at the higher energy. In addition, the
incident angle of the PGE beam was set to 15° from the surface normal to avoid
possible channelling effects in the substrate.
As can be seen from figure 13, the distribution of ranges for a given energy is close to
Gaussian. The concentration n(SN,x) of a trace isotope from an implantation of a
given energy as a function of depth x into the wafer can therefore be approximated
with
∆⋅−−
⋅∆⋅
=2
2
2 2
)(exp
2
)(),(
p
p
p
NN
R
Rx
R
SDxSn
π (3.16)
where D(SN) is the implanted dose [atoms/cm2], Rp is the range, and ∆Rp is the
longitudinal straggling of the trace element.
Rp and ∆Rp can be read out of table 4. In particular the maximal concentration
nmax(SN) of the trace element at depth Rp can be calculated:
2max2
)()(),(
p
NN
pN
R
SDSnRSn
∆⋅==
π (3.17)
3.2.2 Accelerator SIMS results
Depth profiles of the prepared SIMOX wafers were measured with Accelerator SIMS
and are shown in figures 15 to 21. They show the counting rate acquired from the
gated, central 15% of the scanning area of the Cs beam against time. The only
correction made to the data is for dead time. The caesium currents used for the
measurement of depth profiles in the SIMOX wafers were varied from measurement
to measurement between 53 and 290 nA depending on the chosen aperture in the
primary ion source, and the sputter areas were varied between 300 x 300 µm2 and
650 x 650 µm2. This explains the differences in the times needed to sputter through
the Si and the SiO2 layers.
All measurements were performed with a terminal voltage of 5 MV, and carbon foil
strippers were used because of a limitation imposed by the maximal field strength of
the high-energy magnet. On the high-energy side heavy PGE were analysed in charge
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
39
state 9+. According to a model by Sayer, their charge state yield was only ~17%,
whereas the yield of charge state 8+, which is the charge state with the highest
population at 5 MeV, would have been ~21%. Transmission was around 5 per mil for
the heavy PGE. The light PGE were analysed in charge state 8+, which has the
highest charge state yield at 5 MeV, but transmission was similar to that of heavy
PGE. For all elements the atomic ion was injected into the tandem. The PGE-Si- and
PGE-O- secondary molecules were also investigated, but their yields were more than
one order of magnitude smaller than those of the atomic secondary ions.
Due to the high concentrations of PGE in the SIMOX wafers, tuning the system was
no problem. First a calibration setup for the day was obtained by injecting mass
28 amu and analysing 28Si6+ on the high-energy side of the accelerator. The current of
this ion can be measured in a Faraday cup next to the detector. This setup can be used
to calculate the settings of the ion optical elements on the high-energy side for all
other elements to be analysed. For the analysis of PGE, the ion optical elements of the
injector were optimised by measuring the current produced by molecules, such as Si7,
with the secondary electron multiplier just after the tandem. The fragments of such
molecules, for example 28Si2+, can give a counting rate of a few hundred Hertz in the
detector. Tuning the facility to analyse such fragments can be of advantage in order to
briefly check the calibration setting from the analysis of 28Si6+. Its ion optical setup
lies between the calibration setting and those settings needed for the analysis of PGE
and, if necessary, can therefore be used as a second calibration setup. It is then no
problem to predict and find the settings for the PGE of interest.
When analysing a SIMOX wafer, the concentrations of the PGE in the sample are
high enough to produce counting rates that can easily be used for a final fine-tuning of
the setup. This is not always the case. In chapter 4 the analysis of minute
concentrations of iridium in sedimentary layers will be presented. In this case, the ion
optical setup can be checked with clusters containing caesium ions. For example, 133Cs28Si16O2 has a mass of 193 amu and the Cs6+ fragment has an intensity of a few
hundred Hertz when sputtered from an appropriate matrix. The ion optical setup to
focus the Cs6+ beam into the detector is very close to the setup for the analysis of 193Ir9+. Only slight changes in the field strengths of the high-energy electrostatic and
magnetic deflectors are necessary to change to the setup of 193Ir9+. This makes it easier
to check the stability of the measurement during analysis of sedimentary layers.
Technically it was not possible to simultaneously monitor the current of a matrix
element in order to make a correction for fluctuations in the ion currents. This is due
to the large mass differences between silicon and the PGE meaning that a quasi-
simultaneous injection with the beam bouncing system of the low-energy magnet is
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
40
not possible. Possibilities on how to solve this problem will be presented in chapter 6.
However, as can be seen in figure 14 the current of Si6+ measured in a Faraday cup
next to the detector remains stable to within 10% in a given layer of the sample. It has
therefore been assumed that the transmission remains constant throughout the
measurement of a depth profile.
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 500 1000 1500 2000 2500 3000 3500
Time [s]
Si6+
cur
rent
[nA
]
Si
SiO2
Si-Substrate
Fig. 14: Gated 28Si6+ current as a function of time during a depth profile of a SIMOX wafer. The current was measured in a Faraday cup situated next to the gas
ionisation detector. The scanning area of the Cs beam was 450 x 450 µm.
Fig. 15: Depth profile of 195Pt implantation in SIMOX wafer. The counting rate has only been corrected for dead time. The Cs current was 53 nA and the scanning area was
set to 450 x 450 µm.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
41
Fig. 16: Depth profile of 197Au implantation in SIMOX wafer. The counting rate has only been corrected for dead time. The Cs current was 115 nA and the scanning area was
set to 300 x 300 µm.
10
100
1000
10000
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600
Time [s]
Cou
nts
per
10 s
econ
ds
Fig. 17: Depth profile of 193Ir implantation in SIMOX wafer. The counting rate has only been corrected for dead time. The Cs current was 126 nA and the scanning area was
set to 450 x 450 µm.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
42
10
100
1000
10000
0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800
Time [s]
Cou
nts
per
10 s
econ
ds
Fig. 18: Depth profile of 192Os implantation in SIMOX wafer. The counting rate has only been corrected for dead time. The Cs current was 290 nA and the scanning area was
set to 650 x 650 µm.
Fig. 19: Depth profile of 107Ag implantation in SIMOX wafer. The counting rate has only been corrected for dead time. The Cs current was 290 nA and the scanning area was
set to 650 x 650 µm.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
43
10
100
1000
10000
0 300 600 900 1200 1500 1800 2100 2400 2700
Time [s]
Cou
nts
per
15 s
econ
ds
Fig. 20: Depth profile of 103Rh implantation in SIMOX wafer. The counting rate has only been corrected for dead time. The Cs current was 126 nA and the scanning area was
set to 450 x 450 µm.
1
10
100
1000
0 300 600 900 1200 1500 1800 2100 2400
Time [s]
Cou
nts
per
15 s
econ
ds
Fig. 21: Depth profile of 106Pd implantation in SIMOX wafer. The counting rate has only been corrected for dead time. The Cs current was 126 nA and the scanning area was
set to 450 x 450 µm.
The ratio of the total sputter yields of a Si matrix and a SiO2 matrix can be easily read
out of both figure 14 and the depth profiles of the PGE (figures 15 to 21). This is due
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
44
to the fact that on one hand the extracted Si current is different for the 2 matrices
(figure 14) and on the other hand that there is a large increase in the counting rate of
the PGE at the interfaces between the SiO2 and the silicon layers. Averaging over
several measured profiles one calculates that the Cs beam sputters through the 200 nm
thick silicon surface layer in 0.60 of the time it takes it to sputter through the 100 nm
of SiO2. Limited by the cycle time, which was chosen between 10 and 30 seconds,
this value is precise to within 10%. The total sputter yield of a silicon matrix for the
primary beam parameters used for these measurements has been measured by (Ender,
1997a) and found to be 2.4 Si atoms per Cs atom. Together with the atomic densities
of silicon and SiO2 the total sputter yield of a SiO2 matrix can therefore be calculated:
0.34 SiO2 molecules per Cs atom.
Looking at figure 13 and the measured depth profiles it is clear that a significant
fraction of the implanted ions of the lower energy are stopped outside the SiO2 layer.
In the depth profiles this is manifested by the peaks of the counting rate at the
interfaces between the SiO2 and the Si layers. This makes it difficult to evaluate the
depth profiles with methods using the integral of the counting rate from an
implantation peak. A detailed evaluation revealed that a Gaussian fit to the counting
rate in the SiO2 layer can only be used to estimate the maximal counting rate in the
layer. More details on this topic will be presented later on. For the following
evaluation every implantation of a given element and energy is regarded
independently, and only maximal counting rates in the depth profile are compared
with the corresponding concentration maximum derived with the SRIM simulation
allowing one to calculate the Sensitivity Factors and detection limits of PGE in the
corresponding matrix.
Scaled Sensitivity Factors (SSF) are readily calculated using equation 3.10, where the
reference concentration n(S) is the maximal concentration in the implantation profile
as calculated from equation 3.17, but additionally corrected for the isotopic
abundance. Its corresponding counting rate, Iq(SN), is determined out of the depth
profiles for both the SiO2 layer and the Si substrate. Iq(SN) is the maximum of a
Gaussian fit to the counting rate of an implantation peak. Together with the known
primary beam currents, Ip, during the individual measurements the SSF can be
calculated. Table 5 summarises the Scaled Sensitivity Factors for platinum group
elements, gold, and silver in silicon and SiO2. Additionally, the ratio of the two SSF
of a given PGE for the same atomic densities (atoms/cm3) is given indicating the
reduction in absolute sensitivity when changing from one sample matrix to the other.
On average the sensitivity drops by a factor of ~50.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
45
Element Au Pt Ir Os Ag Pd Rh
Cs current [nA] 115 53 126 290 290 126 126
SSF in Si
[(counts/s)/(nA⋅at./cm3)] 6⋅10-18 1⋅10-17 8⋅10-18 1⋅10-18 1⋅10-18 9⋅10-19 2⋅10-18
SSF in Si
[(counts/s)/(nA⋅µg/g)] 0.042 0.081 0.061 0.009 0.015 0.012 0.020
SSF in SiO2
[(counts/s)/(nA⋅at./cm3)] 9⋅10-20 3⋅10-19 1⋅10-19 4⋅10-20 4⋅10-20 (2⋅10-19) (2⋅10-19)
SSF in SiO2
[(counts/s)/(nA⋅µg/g)] 0.0007 0.0026 0.0009 0.0003 0.0006 (0.0025) (0.0036)
PGESiO
PGESi
2SSF
SSF 66 35 75 30 29 (5) (6)
Table 5: Accelerator SIMS Scaled Sensitivity Factors of PGE for Si and SiO2 matrices.
From figure 14 it is also possible to determine a SSF for sputtered Si ions. The
average Si current sputtered from the SiO2 layer is 0.15 nA and the Si current coming
from the Si substrate is 4.0 nA. Taking into account that the Si ions are analysed in
charge state 6+ one gets:
)/./()/(102.1
)/./()/(105.1317
316
2cmatnAscountsSSF
cmatnAscountsSSFSi
SiO
SiSi
⋅⋅=
⋅⋅=−
−
Following the discussion in section 3.1.3, it is now possible to calculate the Scaled
Sensitivity Ratios (SSR) of the PGE relative to Si6+ ions from the Si and SiO2
matrices. These are summarised in table 6 together with the ratios of the SSR in the
different matrices. In comparison to table 5 the ratio of the SSR indicates the relative
difference of the sensitivities when analysing a given PGE in these matrices. As can
be seen the reduction is less than one order of magnitude. This means that most of the
sensitivity is lost due to a reduced probability of the formation of negative secondary
ions in the sputter process caused by the presence of oxygen in the sample matrix.
Generally, this means that large variations in sensitivities have to be expected when
analysing different matrices.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
46
Element Au Pt Ir Os Ag Pd Rh
SSR in Si 0.038 0.073 0.054 0.008 0.008 0.006 0.010
SSR in SiO2 0.008 0.027 0.009 0.003 0.004 (0.014) (0.020)
PGESi,SiO
PGESi,Si
2SSR
SSR 5.0 2.7 5.9 2.3 2.2 (0.4) (0.5)
Table 6: Accelerator SIMS Scaled Sensitivity Ratios of PGE and Si ions analysed in Si and SiO2 matrices.
Another interesting value that can be estimated from the measured data is the
detection limit of a PGE in the two matrices. Again, using the Gaussian fit to the
measured counting rate and the corresponding simulated maximal trace element
concentration of the implantation and assuming a background counting rate of
0.01 Hz for all PGE (except for Au, where 1 Hz is a realistic background counting
rate) the detection limit, DL, can be calculated from:
)S(r
)S(n
)S(I
Hz01.0)S(DL
N
Nmax
Nq⋅= (3.18)
Normalising the counting rate, Iq(SN), to the full scanning area and to a primary
Cs beam current of 500 nA before inserting into equation 3.18, gives the detection
limits for PGE in Si and SiO2 as presented in table 7. Again, the effect of the smaller
negative ion yields from the SiO2 matrix can be seen in the worse detection limits.
Element Au Pt Ir Os Ag Pd Rh
Detection limit in Si
[at./cm3] 3⋅1014 5⋅1012 4⋅1012 4⋅1013 3⋅1013 8⋅1013 1⋅1013
Detection limit in Si
[at./Si atom] 7⋅10-9 1⋅10-10 8⋅10-11 8⋅10-10 7⋅10-10 2⋅10-9 3⋅10-10
Detection limit in Si [ng/g] 48 0.7 0.5 5 3 6 1
Detection limit in SiO2
[at./cm3] 2⋅1016 2⋅1014 3⋅1014 1⋅1015 9⋅1014 (4⋅1014) (8⋅1013)
Detection limit in SiO2
[at./SiO2] 9⋅10-7 7⋅10-9 1⋅10-8 5⋅10-8 4⋅10-8 (2⋅10-8) (3⋅10-9)
Detection limit in SiO2
[ng/g] 2800 23 36 150 65 (30) (6)
Table 7: Accelerator SIMS detection limits of PGE in Si and SiO2.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
47
Looking at the depth profiles of Pd and Rh in figures 20 and 21 it seems as if the
concentration maximum of the 600 keV implantation lies close to the interface
between the SiO2 layer and the silicon substrate. This is probably due to a channelling
effect during implantation of the trace elements. The fit of the Gaussian distribution to
the counting rate in the SiO2 layer is therefore not reliable and distorts SSF, SSR and
detection limits derived from the depth profile for these two elements in this matrix.
This is why the values for Pd and Rh in a silicon dioxide matrix have been set in
parentheses.
Before concluding, a word should be said about the reliability of the presented
numbers. First of all, the depth profiles shown in figures 15 to 21 were not all
measured on the same day. Variations in the transmission from day to day can be
significant. The fluctuations in the transmissions due to this are not expected to be
larger than a factor 2 - 3.
In addition, it should also be kept in mind, that the concentration maxima of the
implantation profiles simulated with TRIM probably overestimate the real
concentrations. This is because the longitudinal straggling of heavy elements in the
energy range applicable here is underestimated by TRIM due to the difficult
prediction of nuclear stopping. This results in the assumption of a narrower Gaussian
concentration distribution of the trace isotope and therefore an overestimation of the
maximal concentration of the profile. A detailed evaluation of the depth profiles
revealed that the TRIM prediction of the range Rp is accurate to with 20%. This was
checked by calculating the sputter rate with the time needed to erode through the
200 nm thick surface layer. With this information the depth, Rp, at which the substrate
implantation is located can be calculated with the time it takes to reach the maximum
in the counting rate after having passed the second interface assuming that the second
interface lies at a depth of 300 nm. This also gives a depth scale to which one can
compare the width of the Gaussian fit to the depth profile. It turned out that TRIM
underestimates the longitudinal straggling by 20 – 50 %. Therefore, the simulated
concentrations are overestimated by 20 – 50% and the Scaled Sensitivity Factors
given here are to be regarded as lower limits whereas the detection limits can be
regarded as upper limits to the true values.
The widths of Gaussian distributions fitted to the depth profiles in the region of the
SiO2 layer, however, didn’t correspond to the predictions made by TRIM. The fits
only reproduced the longitudinal straggling to within a factor of 2. This is also due to
the fact that the depth profiles are difficult to fit in this region, due to the presence of
the large interface peaks. Therefore, only the maximal counting rates measured in the
SiO2 layer were compared to the TRIM simulations.
Chapter 3 Accelerator SIMS Analysis of a SiO2 Matrix
48
Also, the assumed background counting rate of 0.01 Hz in equation 3.18 is only a
practical assumption. In many cases virtually no background was measured over a
period of up to 20 minutes!
In total, all SSF, SSR and detection limits given here are correct to within about a
factor of 5 and are only meant to demonstrate the capabilities of Accelerator SIMS.
49
Chapter 4 Platinum Group Elements at the KT-Boundary
As announced earlier on, this chapter deals with the application of the insights gained
in chapter 3 to in-situ bulk concentration analysis with Accelerator SIMS in the field
of geology. The advantages of Accelerator SIMS in comparison to other analytical
methods will be demonstrated on the measurement of platinum group elements (PGE)
in sedimentary layers at the transition from the Cretaceous to the Tertiary period (KT-
boundary). In particular, the measurement of a concentration peak of iridium (56 ng/g
at the maximum) at the transition line between the two periods will be looked at. This
is a perfect topic to test the abilities of Accelerator SIMS, since the processes involved
in evolutionary history at the transition from the Cretaceous to the Tertiary have
received much attention over the last two decades resulting in a wealth of information
to compare Accelerator SIMS test measurements to. Accelerator SIMS measurements
reproduce data gained with Neutron Activation Analysis (NAA) very nicely. But the
lateral resolution of Accelerator SIMS is orders of magnitude higher (~100 µm
instead of ~1 cm) making it possible to study trace elements in sedimentary layers at a
much smaller lateral scale.
4.1 Introduction to the KT-Boundary
In 1980 Luis Alvarez and his co-workers proposed that the mass extinction at the end
of the Cretaceous period, 65 million years ago, was provoked by a comet or asteroid
of some ten kilometres in diameter colliding with Earth. This novel explanation of the
observed anomalies at the transition between the two periods was only based on a
peak in the iridium concentration at the transition line in the corresponding
sedimentary layers, supposedly material from the extraterrestrial projectile. The
theory stands in opposition to the proposition that massive volcanic eruptions also
could have transported dust from the Earth’s mantle, containing an excess of iridium
relative to the Earth’s crust, into the atmosphere to form a concentration peak of
iridium in sedimentary layers. The debate triggered a worldwide search for the impact
crater and other evidence to support the theories (Goss Levi, 1992). Today, a lot more
evidence has been gathered and - even if the KT-debate is still ongoing - it is believed
that a structure beneath the Yucatàn peninsula in the Gulf of Mexico, the so-called
Chapter 4 Platinum Group Elements at the KT-boundary
50
Chicxulub crater, is the remains of the impact site. Its age was determined by 40Ar/39Ar-dating to 64.98 ± 0.05 Ma, which is a strong indication that the crater is
related to the KT-boundary (Swisher et. al, 1992).
Whatever the theory, a significant amount of PGE rich dust was blown into the
atmosphere and distributed globally before it fell out, creating a small excess of
platinum group elements in sedimentary layers as they formed at that time. The
element abundances and isotope ratios of the PGE in the sedimentary layers are
therefore a key to answering the question as to whether a meteorite struck Earth of
whether volcanic eruptions lead to the climate change resulting in the mass extinction
at the end of the Cretaceous.
From a meteorite fragment found in the KT-boundary sediments in the Pacific Ocean
it has been anticipated that the asteroid was a chondrite of type CV, CO, or CR (Kyte,
1998). These chondrites carry a high abundance of platinum group elements
(~700 ng/g of Ir, ~1000 ng/g of Pt, and others) in comparison to the Earth’s crust
(~1 ng/g of Ir, ~5 ng/g of Pt). Therefore, not only a peak in the concentration of
iridium is expected in sedimentary layers, but a peak in the platinum concentration
would be expected as well. Alvarez determined an iridium peak concentration of
around 42 ng/g in samples found in Denmark, a concentration that - according to the
results of chapter 3 - should be measurable with Accelerator SIMS in an adequate
sample matrix.
4.2 Sample Description
The samples used for the Accelerator SIMS experiments were provided through the
courtesy of Dr. Beda Hofmann, Natural History Museum Bern. They were taken from
sedimentary layers at the Starkville South section, Raton basin, Colorado. Figure 22
shows a picture of the sampling site with a Swiss Army Knife on the right hand side
of the picture to illustrate the size scale. The light layer, in front of which the knife has
been placed, is kaolinitic clay typical for the sedimentary structure of the KT-
boundary in Northern America. In Alvarez’ theory this is basically material that fell
out immediately after the impact (Pillmore et al., 1987). Immediately above it is a
slightly darker and redder layer of finer grain. This is predominantly dust that fell out
during the years after impact and should contain the highest fraction of extraterrestrial
material.
Chapter 4 Platinum Group Elements at the KT-boundary
51
Fig. 22: Excavation site at the Starkville South section, Raton basin, Colorado. The KT-boundary samples used for Accelerator SIMS experiments came from this site. A Swiss Army Knife on the right indicates the size scale.
4
4
3
2
1
5
25 m
m
Fig. 23: Continuous cross section of the sedimentary layers around the KT-boundary from Starkville, Colorado. 1-2: Carbonaceous shale of the Cretaceous; 2-3: Kaolinitic clay layer (immediate impact fall-out); 3-4: KT-horizon (material of very fine grain containing post-impact dust fall-out); 4-5: Coal layer of the Tertiary.
About one millimetre thick cross sections were cut out of the layer structure described
above and glued onto glass object slides. The region of interest was distributed over
two slides. Figure 23 shows a photograph of these slides. Together they represent a
continuous section of the sedimentary layers starting at the carbonaceous shale of the
Cretaceous (region 1-2) followed by the layer of caolinitic clay (2-3). The KT-horizon
consisting of post-impact dust sediments lies between 3-4, and 4-5 is the coal layer of
Swiss Army Knife
Chapter 4 Platinum Group Elements at the KT-boundary
52
the early Tertiary period. The iridium peak is expected to be between 3-5 (Hofmann,
1999).
Unfortunately, both object slides had to be cut to size after the samples had already
been glued on to them in order for the samples to fit into the sample holder of the
Accelerator SIMS chamber. During this process the smaller sample of the Tertiary
layer split perpendicular to the direction of sedimentation. This, however, did not
affect the measurements. As can be seen in figure 24, which shows the samples
prepared for analysis, it sufficed to press the two fragments as close together as
possible and not to measure in the direct vicinity of the crack between the two halves.
In order to prevent electrical charging of the samples by the primary beam, the
samples were coated with a 50 nm thick carbon layer. This gives the samples the
silvery look in figure 24. Unfortunately, it turned out that the carbon layer contained a
platinum contamination and a reliable measurement of the platinum concentration in
the sediments was not possible. This would have been of great interest, because of the
high platinum abundance in CV, CO and CR chondrites.
4.3 Measurements and Results
Accelerator SIMS measurements were performed with the same analytical setup as
the measurements presented in chapter 3. The terminal voltage was set to 5 MV and
carbon foil strippers were used. The low-energy mass spectrometer was tuned to inject
mass 193 amu and on the high-energy side of the tandem accelerator 193Ir9+ was
analysed. The machine was tuned with iridium implantations in a silicon matrix
before measurements on the sedimentary layers were performed. Detailed information
on the tuning procedure was given in section 3.2.2.
The main interest was in reproducing the lateral concentration distribution of iridium
in the sedimentary layers (Pillmore et al., 1987) and all Accelerator SIMS
measurements were performed along the dashed white line in figure 24. In order to
measure the low concentrations of iridium the primary caesium current had to be
maximised. For this reason a beam spot size of ~100 µm was chosen, which is also a
lot larger than the size of the mineral grains in the samples (~1 µm). Therefore, the
measurement was not influenced by the microscopic structure of the samples.
It turned out that iridium could only be found in the coal layer of the Tertiary. For this
reason the matrix composition of the sedimentary layers was analysed with
Rutherford Backscattering Spectrometry (RBS). This way the main matrix
components could be easily identified and a possible change in the secondary ion
Chapter 4 Platinum Group Elements at the KT-boundary
53
yield due to changes in the matrix composition across the sample monitored. The RBS
analyses were performed in 1 mm steps along the same dashed line indicated in
figure 24 and the results are summarised in figure 25 together with the
Ir concentrations measured with Accelerator SIMS.
5
6
4
43
2
1
Fig. 24: Pictures of the carbon coated samples. The white dotted line indicates the line of analysis along which the Accelerator SIMS and RBS analyses were performed. The thin black lines and dots next to the dashed line are sputter marks from the caesium beam of the Accelerator SIMS ion source. The numbers on the right correspond to the caption of figure 23. Additionally, no. 6 indicates the crack along which the smaller sample broke.
One can clearly see that in the coal layer of the Tertiary carbon is the dominant matrix
element with an atomic fraction of more than 60%. At the transition line to the
sediments of the KT-boundary the carbon concentration drops sharply and the oxygen
concentration on the other hand rises. This rise in the oxygen abundance is caused by
the increasing presence of silicon and aluminium. The concentration of aluminium is
not shown in figure 25, but the oxygen abundance goes hand in hand with a mixture
of SiO2 and Al2O3 of about 2:1 in the matrix. It is also nicely demonstrated that within
a given sedimentary layer the matrix composition does not vary and stays constant. In
the layer of the carbonaceous shale of the Cretaceous the carbon concentration rises
again and the oxygen and with it the SiO2 and Al2O3 compounds diminish.
Fig. 26
Chapter 4 Platinum Group Elements at the KT-boundary
54
Fig. 25: Summary of the atomic concentration measurements performed on the sedimentary layers at the KT-boundary. Iridium was measured with Accelerator SIMS and carbon, silicon and oxygen abundances were determined with RBS.
0 1 2 3 4 5 6
05
1015
2025
30
Position [mm
]
0 10 20 30 40 50 60 70 80 90 100
Relative atomic concentration [%]
Ir Concentration [ng/g]normalised to a graphite matrix
Tertiary
Oxygen
Kaolinitic
clay layerC
reta-ceous
KT
-H
orizon
Carbon
Silicon
Ir
0 1 2 3 4 5 6
05
1015
2025
30
Position [mm
]
0 10 20 30 40 50 60 70 80 90 100
Relative atomic concentration [%]
Ir Concentration [ng/g]normalised to a graphite matrix
Tertiary
Oxygen
Kaolinitic
clay layerC
reta-ceous
KT
-H
orizon
Carbon
Silicon
Ir
Chapter 4 Platinum Group Elements at the KT-boundary
55
In the light of this change of matrix composition it becomes clear why no iridium
could be detected with Accelerator SIMS in the kaolinitic clay layer. According to the
results of chapter 3, the sensitivity of Accelerator SIMS with respect to iridium drops
in a SiO2 dominated matrix by almost two orders of magnitude. In such a matrix even
the highest expected iridium concentration of 56 ng/g lies close to the detection limit
of Accelerator SIMS (table 7 in chapter 3). Carbon, however, has much more the
same properties as a silicon matrix and the detection limit of Ir is therefore much
lower. This is why iridium is detected in the carbonaceous layer of the Tertiary even
though the iridium concentration is much lower there. Figure 25 shows four iridium
concentration measurements in the coal layer. It nicely reproduces the form of the
iridium concentration profile published by Pillmore et al., 1987, which lies between 1
and 10 ng/g in the layer under consideration.
It would also be possible to perform the Accelerator SIMS measurements with a
lateral resolution corresponding to the beam spot size of around 100 µm. To
emphasise this, figure 26 shows a magnification of a section of figure 24 The thin
black lines are marks made by the caesium beam of the Accelerator SIMS source.
They were made by moving the sample in 150 µm steps under the stationary caesium
beam with a beam diameter of about 100 µm (the largest aperture of the ion source
was used). One can clearly see the sputter spots of the individual steps meaning that
the beam spot size of the caesium beam is significantly smaller than the step size.
Fig. 26: Magnification of the area marked in figure 24 indicating the sputter marks made by the Accelerator SIMS primary beam.
3 mm
Chapter 4 Platinum Group Elements at the KT-boundary
56
The absolute iridium concentration in the coal layer was determined by producing a
standard with the implantation technique described in chapter 3. An 193Ir fluence of
1.0⋅1013 cm-2 was implanted into a graphite plate at an energy of 1 MeV. Graphite is a
suitable approximation to the matrix of the carbonaceous layer, and it is expected that
the secondary ion yield of iridium from graphite is close to its yield from the
carbonaceous sediment layer.
0
1000
2000
3000
4000
5000
6000
7000
8000
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Time [s]
Cou
nts
per
15 s
econ
ds
data Gaussian fit
Fig. 27: Depth profile of a 1 MeV Ir implantation in graphite together with a Gaussian fit to the data.
Following the procedures used in chapter 3 the range and longitudinal straggling of
the profile were simulated with TRIM 2000 and with equation 3.17 the maximal
concentration of the profile can be estimated. The simulation gave a range, Rp, of
221 nm and a longitudinal straggling, ∆Rp, of 31 nm. This gives a maximal
concentration of 1.3⋅1018 Ir atoms/cm-3. An Accelerator SIMS depth profile of the
implantation was measured prior to the analysis of the cabonaceous layer. The
measured maximal counting rate was then used as a reference for concentration
calibration taking into account that depth profiles are evaluated out of the central 15%
of the scanning area of the caesium beam. The iridium concentrations presented in
figure 25 were calculated by comparing the maximal counting rate of the profile to the
counting rates of the measurements on the carbonaceous sediment layer. Figure 27
shows the measured profile together with a Gaussian fit. As discussed in chapter 3,
the range straggling of the implantation is probably higher than the value calculated
with TRIM. This can also be seen in figure 27. Assuming, according to the
Chapter 4 Platinum Group Elements at the KT-boundary
57
simulation, that the maximal counting rate was measured at a depth of 221 nm, then
the variation of the concentration distribution is 46 nm. For the same reason as
mentioned in chapter 3 this results in an additional systematic error of the
concentration measurements of 50%.
For comparison, by using the same procedure as explained in chapter 3, one calculates
a detection limit of 1.5⋅1012 cm-3 (1.3⋅10-11 Ir atoms/C atom or 0.2 ng/g) for iridium in
graphite.
4.4 Conclusions
It has been demonstrated that Accelerator SIMS is an analytical method suited to
perform trace element analysis of platinum group elements in ng/g-concentrations in
geological samples with a lateral resolution of ~100 µm. In well-suited matrices, such
as carbonaceous layers, even sub-ng/g detection limits can be reached. This represents
an improvement to in-situ bulk concentration analysis, which before could not be
performed at such low concentrations with such high lateral resolution. Scientific
problems at this lateral scale can be studied now. The value of such analytical
methods is also apparent in the light of the recent observation of another iridium
anomaly at the transition between the Triassic and Jurassic periods (TJ-boundary) that
might have the same origin as the iridium anomaly at the KT-boundary (Olson et al.,
2002).
To date, the analyses at the KT-boundary have been performed with Neutron
Activation Analysis (NAA) and laser ablation ICP-MS (Inductively Coupled Plasma
Mass Spectrometry) for iridium (Alvarez et al., 1980; Lichte, 1992). AMS
measurements of the concentrations of platinum and iridium in sedimentary layers
have also been performed by Rucklidge et al., 1982, and isotopic ratios of PGE in
rocks have been measured by Chew et al., 1984. But these methods have the
disadvantage that either they cannot measure with the same lateral resolution
achievable with the PSI/ETH Accelerator SIMS apparatus (NAA, AMS) or the
sensitivity of the method is impaired by molecular interferences (LA-ICP-MS). Also,
in contrast to NAA, Accelerator SIMS is more or less equally sensitive to all PGE,
therefore it is a universal method with higher sensitivity and higher lateral resolution
for PGE analysis in sedimentary layers. The analysis of other trace elements also has
to be considered.
A more detailed comparison of Accelerator SIMS to other analytical methods will be
made in chapter 6.
Chapter 4 Platinum Group Elements at the KT-boundary
58
Monitoring the composition of the matrix, however, is a must when analysing
heterogeneous samples. This also poses the problem of finding a suitable standard
sample. If this is not possible, the implantation of the trace element of interest directly
into the sample and measuring a depth profile of the implantation is a method to
produce a concentration calibration. For trace element concentration measurements it
should generally be possible to reach an accuracy of 40% and better.
59
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
In chapter 4 the limits of Accelerator SIMS with respect to in-situ analysis of trace
element concentrations were explored. When wanting to analyse small samples, such
as individual mineral grains, it is clear that the limit to the size of an analysable
mineral grain in a thin section is set by the beam spot size of the primary caesium
beam. This restriction is removed when a mineral grain is removed out of its native
rock and is set into a pure matrix that will not interfere during trace element analysis.
This method requires more effort to prepare the sample for analysis, but on the other
hand objects far smaller in size can be analysed.
In this chapter the limits of the required sample size for an Accelerator SIMS
measurement shall be explored. However, the following discussion contains more
than just simply exploring the limit to the size of a sample in order to make a
comparison with other analytical techniques. The work presented here is basically a
new method for routine measurement of natural 10Be/9Be ratios in ferromanganese
crusts and will be presented in this way. The sample sizes that will be regarded here
are in the range of 100 ng of beryllium, a total sample size that so far has never been
measured with AMS. The application of this new technique to the analysis of carrier-
free samples of other radioisotopes routinely analysed with AMS is therefore also a
prospect for the future.
By comparing results from test samples from ferromanganese crusts it will become
clear that precision and sensitivity of this new Accelerator SIMS method is higher
than with methods previously used to gain the same information. These methods are
on one hand two separate measurements of the 10Be and 9Be concentrations with AMS
and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) respectively, and on
the other hand a method developed on an ISOLAB-120 SIMS instrument in Oxford
(Belshaw et al., 1995).
5.1 Introduction
The cosmogenic nuclide 10Be has found many applications as a tracer in the
environmental sciences over the last two decades and is one of the most frequently
measured radionuclides with AMS. Although it is mostly the 10Be concentration in the
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
60
original sample that is of interest, there are applications in which the abundance of
cosmogenic 10Be relative to terrestrial 9Be provides important information (von
Blanckenburg et al., 1996b; Robinson et al., 1995). One such application that will be
addressed in the following is the measurement of the 10Be/9Be ratio as a function of
depth in ferromanganese crusts. With this information growth rates of crusts are
determined and past oceanographic events reflected by changes in the radiogenic
isotope composition in the crusts are dated (Ling et al., 1997; Frank et al., 1998).
Because of the relatively low concentration of 9Be in ferromanganese crusts (1 –
10 ppm) sample sizes are too small ( ~100 ng of 9Be) to be analysed with methods
presently used in conventional AMS. But due to the high sensitivity of AMS several
hundred micrograms of 9Be carrier can be added to the sample during chemical pre-
concentration bringing the sample size into the range measurable with AMS.
However, this way information on the natural 10Be/9Be ratio is lost and can only be
obtained by performing an additional measurement of the 9Be concentration in the
sample, for example with ICP-MS, which introduces additional uncertainty in the
determination of the 10Be/9Be ratio. Therefore, apart from a more simple chemical
preparation of the sample, it was also anticipated that a direct measurement of the
natural 10Be/9Be ratio would be more precise than two individual measurements.
A method based on SIMS for the direct measurement of the natural 10Be/9Be ratio has
been developed by a group in Oxford (Belshaw et al., 1995). The detection of 10Be+
with SIMS, however, is hampered by the presence of 9BeH+ and 10B+ in the secondary
ion beam. Because of the use of an accelerator, Accelerator SIMS has additional
possibilities of suppressing these interferences. The extension of the range of
measurable ratios towards smaller ratios is therefore not to be excluded.
When measuring carrier-free samples with the focussed caesium beam of the
Accelerator SIMS source the resulting secondary ion currents are about three orders
of magnitude smaller than those produced by high current sources used for
conventional 10Be AMS measurements. Because 10Be/9Be ratios in carrier-free
samples are also about three to four orders of magnitude higher, this means that three
to four orders of magnitude smaller amounts of beryllium oxide can be analysed for
long enough to measure the natural 10Be/9Be ratio to a precision of a few per cent.
5.2 Sample Preparation and Loading
For each sample 30 – 130 mg of material was taken from the ferromanganese crust.
Chemical purification and preconcentration of the Be followed a previously published
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
61
method (von Blanckenburg et al., 1996a) with some modifications, mainly a
miniaturisation and a reduction to the first two column separation steps and was
gratefully performed under the supervision of Martin Frank, Institute for Isotope
Geology and Mineral Resources, ETH Zurich. The detailed recipe of the procedure
can be found in Appendix A. It enables a high-purity separation of nanogram amounts
of beryllium from the matrix of the ferromanganese crust. At the end of chemical
pretreatment the samples are evaporated in small Teflon vials.
As we have seen in chapter 2, the design and geometry of the sample stage require
that only objects of wafer-like shape with a plane surface of about 20 x 20 mm can be
mounted onto the sample stage in the sputter chamber. Following dissolution in 1-3 µl
of 65% HNO3 the samples were pipetted onto stainless steel wafers held at a
temperature of 65°C. The dimensions of the wafer are 24 mm x 24 mm x 0.1 mm and
stainless steel was chosen because of its hygroscopic surface allowing the pipetted
drops to disperse on the surface thereby distributing the beryllium evenly over the
wafer after the solvent has evaporated. Without further precautions the resulting spots
of beryllium are 2 – 3 mm in diameter. The beryllium is deposited on the wafer as
BeNO3 and by baking it at 850°C for two hours the beryllium nitrate decomposes to
beryllium oxide. A second reason for the choice of stainless steel is its resistivity
against high temperatures and the solvent acid. The wafers are therefore resistant to
corrosion during the baking process.
For sample sizes of ~100 ng the area over which the drop disperses has to be
controlled by creating a hydrophobic ring around a hygroscopic area of about 1 mm
diameter. This is done by heating the wafer to 225°C and then drawing a ring by hand
on the surface of the wafer with the end of a plastic capillary tube usually used for
pipetting µl-amounts of liquid. 225°C is sufficient for the plastic to change the
hygroscopic properties of the steel surface, but still low enough for the plastic not to
melt completely. The plastic ring subsequently evaporates in the baking process.
Controlling the size of the area over which the sample is allowed to disperse is
important, because the beryllium has to be distributed evenly over the surface of the
wafer but at the same time a certain thickness of the beryllium layer is necessary. The
analysis time available before the primary beam has sputtered through the beryllium
layer and has to be moved to a fresh spot on the sample increases with the thickness of
the layer. On the other hand mounds or an uneven structure of beryllium on the plane
surface of the stainless steel will cause different ion optical conditions for different
positions of the primary beam on the sample. The mass fractionation that is caused by
this can be as large as a few percent.
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
62
For this experiment only one unknown sample was pipetted onto a single stainless
steel wafer. In addition, standards and blanks were pipetted onto the same wafer as the
unknown sample. Due to the larger amounts of beryllium in standards and blanks the
hydrophobic ring on the steel surface is not necessary. The standard used for the
measurements presented here is the M3 fraction of the dilution series published by
Hofmann et al., 1987, with a nominal 10Be/9Be ratio of 3.64⋅10-8. This standard is
dissolved in 0.1 N HCl, an acid that readily corrodes stainless steel during the baking
process. To reduce this corrosion, the standard consisting of 1 µl of original
M3 solution was evaporated at 150°C in a Teflon vial before being redissolved in 2 µl
of 65% HNO3 and transferred to the wafer. 1 µl of the blank solution (1000 ppm 9Be
carrier solution routinely used for 10Be AMS) was pipetted directly onto the wafer.
Because the beryllium is dissolved in 0.5 N HNO3 no pretreatment of this solution
was necessary.
Since the beam spot of the primary beam is a lot smaller than the size of the sample,
the electrically insulating beryllium oxide has to be made conducting so that charging
of the sample is not a problem during analysis. To achieve this, the entire stainless
steel wafer with the samples on it is covered with a 20 nm thick, electrically
conducting layer by evaporating high purity gold onto it.
Fig. 28: Sample mounting procedure. Last steps before measurement.
Sample dissolved in 2 µlof 65% HNO3 and pipetted on to a stainless steel wafer
Evaporate solvent at 65°C
Dry sample by heating to 120°C for 2 hours
Bake at 850°C for 2 hours(BeNO3 Ø BeO + NO2)
Coat wafer and sample with 20 nm of gold
Stainless steel wafer
AuBeO
1 mm
Stainless steel wafer
BeNO3 +H2O
1 mm
Hydrophobic plastic filmto confine the sample
Sample dissolved in 2 µlof 65% HNO3 and pipetted on to a stainless steel wafer
Evaporate solvent at 65°C
Dry sample by heating to 120°C for 2 hours
Bake at 850°C for 2 hours(BeNO3 Ø BeO + NO2)
Coat wafer and sample with 20 nm of gold
Stainless steel wafer
AuBeO
1 mm
Stainless steel wafer
AuBeO
1 mm
Stainless steel wafer
BeNO3 +H2O
1 mm
Hydrophobic plastic filmto confine the sample
Stainless steel wafer
BeNO3 +H2O
1 mm
Hydrophobic plastic filmto confine the sample
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
63
Figure 28 schematically summarises the procedure of mounting samples on stainless
steel wafers. Before measurement the shapes and sizes of the samples are observed
with a pair of binoculars in order to recognise and hold back samples that are likely to
be problematic to measure. Especially, samples that are too small for measurement
(<50 ng) can be held back.
With this sample mounting technique cross contamination between the sample to be
measured, the standards and the blanks on the steel wafer is possible. The obvious
places where cross contamination cannot be controlled are the baking of the wafer in
the oven and during the actual measurement. At temperatures of 800°C more volatile
beryllium compounds can degas from the sample and diffuse to another sample on the
same wafer. On the other hand, during measurement beryllium that is sputtered from
the sample as neutrals will be deposited on the surfaces of electrodes close to the
sample. They can return to the wafer as tertiary particles sputtered by not-extracted
secondaries and possibly contaminate samples on it. These contamination channels
have been investigated by loading wafers with either only blanks (blank ratio
measured by conventional 10Be AMS: ~10-15) or blanks together with standards with 10Be/9Be ratios in the range of 10-7-10-9. It was found that the background ratio is
better than a few times 10-13 for blanks on a wafer holding only blanks (95%
confidence level for zero 10Be counts recorded during the charge collection
corresponding to ~1013 9Be atoms). However, if the wafer has been prepared with a
standard on it, the background ratios of the blanks are three orders of magnitude lower
than the measured ratio of the sample irrespective of whether the blank is the first
sample to be analysed on a given wafer or not. Cross contamination therefore occurs
during the baking process in the oven. However, if the ratios of the sample to measure
and the standard do not lie more than two orders of magnitude apart then this should
not impose a problem for the measurement.
5.3 Instrumentation
The experimental setup used for the measurement of carrier-free samples is different
from the one used for the measurements presented in chapters 3 and 4. However, the
only difference between the setup of a routine 10Be AMS measurement performed at
the PSI/ETH AMS facility and the measurement of carrier-free 10Be samples is the
use of a different ion source. BeO- ions are extracted from the ion source and the
switching system of the low-energy magnet injects mass 25 amu and 26 amu
sequentially into the tandem accelerator, which is run at 5.6 MV with a gas stripper.
Like in routine measurements the 16O5+ fragment from the break-up of the 9Be16O-
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
64
molecule is analysed and used for normalisation on the high-energy side of the
accelerator. The O5+ current is measured in a Faraday cup situated behind the
analysing magnet. The 10Be3+ beam additionally passes a second stripping foil and a
second magnetic deflection to reduce the isobaric interference 10B due to a different
charge state distribution at the higher energy. The remaining 10B in the beam is
stopped in a gas absorber just in front of the gas ionisation detector.
Fig. 29: Relevant elements for the measurement of carrier-free beryllium samples on the low-energy side of the PSI/ETH AMS facility.
Fig. 30: Relevant elements for the measurement of carrier-free beryllium samples on the high-energy side of the PSI/ETH AMS facility.
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
65
For clarification figure 29 shows the relevant elements of the low-energy side of the
PSI/ETH AMS facility and figure 30 shows the equivalent on the high-energy side of
the tandem accelerator.
When setting up the facility for carrier-free 10Be AMS, the high-current source is used
first to tune the facility identically as for a routine 10Be AMS run. Its cathode potential
is then set to match the sample potential of the Accelerator SIMS source. Once the
facility is tuned the high current source is turned off and the electrostatic deflector just
after the source (ED 3) is retracted out of the beam line. Also, the integrator used to
measure the 16O current has to be changed to one suitable for the measurement of
100 pA-currents and the pulse of the beam bouncing system, during which 9Be16O- is
injected into the accelerator, is increased from 200 µs to 20 ms. The extraction unit of
the Accelerator SIMS source is roughly tuned by maximising the current measured in
a Faraday cup before the low-energy analysing magnet. Fine-tuning of the extraction
is done by maximising the current of the fragments of the injected 9BeO- ions in the
Faraday cup following the accelerator and after that in the Faraday cup following the
high-energy analysing magnet. Except for minor retuning of the low-energy magnet,
none of the settings of the ion optical elements tuned with the high-current source are
changed during measurement.
With the facility tuned and ready for analysis, the stainless steel wafers prepared
according to section 5.2 are brought into the Accelerator SIMS source one by one.
With the x/y-stage the sample to measure is placed under the primary beam and
extraction of the secondary ions is optimised by adjusting the steerers and lenses in
the extraction of the source. If possible, the extraction unit is left unchanged until the
next wafer is brought into the chamber thus assuring identical analytical conditions
for all the samples on a given wafer. However, the tuning of the extraction unit should
be checked before every measurement in order to minimise the possibility of mass
fractionation during analysis due to a not-quite-optimal extraction of the secondary
ions.
The measurement procedure of a wafer is as follows. First, a blank sample is
measured to determine the background. Then, alternately, the sample and the standard
are measured three times for approximately 10 minutes each. Figure 31 illustrates a
typical measurement of 10 minutes length by plotting the 10Be counting rate and 16O5+ current together with the resulting 10Be/9Be ratio as a function of time. For
technical reasons this particular measurement was performed with a Cs+ current of
about 100 nA. The strong increase in current after about 450 seconds is caused by
moving the sample under the caesium beam by a few tens of micrometers with the
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
66
stepping motors of the x/y-stage. The measured ratio is not influenced by this increase
in current.
Fig. 31: Typical run of a sample showing the 10Be counting rate and the 16O5+ current as well as the resulting 10Be/9Be ratio as a function of time. For technical reasons the Cs+ current during this measurement was about 100 nA
Finally the blank is measured again to monitor possible changes in the background of
the measurement.
Taking the weighted mean of the three measurements gives the final 10Be/9Be ratio of
the sample. Samples with a 10Be/9Be ratio of 10-9 can be measured to a precision of
about 3%. Counting statistics is the limiting factor in the uncertainty, and in the
limited amount of data available at present no external error is observed. With a
Cs+ current of 600 nA, the 16O5+ current is typically 200 - 300 pA with a yield of
secondary BeO- ions of about 1%. This yield is comparable to the yields of other ion
sources (1-2 %). The 30 minutes of analysis time mentioned above do not use up all
of the sample. Under analytically acceptable conditions analysis time can be extended
up to 1.5 hours, which makes the measurement of 10Be/9Be ratios in the 10-10 range
possible to an accuracy of around 10%. Not all of the sample beryllium can be
consumed during measurement and the overall useful yield of a single sample is
around 4⋅10-4. Background ratios are ~3⋅10-11 when normalising to standard solution
M3.
0
10
20
30
40
50
60
70
80
0 100 200 300 400 500 600 700
Time [seconds]
Cou
nts
of
10B
e pe
r 30
sec
onds
and
16O
5+cu
rren
t [pA
]
0
10
20
30
40
50
60
10B
e/9B
e [x
10-9
]
10Be counts 16O5+ current 10Be/9Be ratio
weighted mean ratiowith 1σ−error of mean
0
10
20
30
40
50
60
70
80
0 100 200 300 400 500 600 700
Time [seconds]
Cou
nts
of
10B
e pe
r 30
sec
onds
and
16O
5+cu
rren
t [pA
]
0
10
20
30
40
50
60
10B
e/9B
e [x
10-9
]
10Be counts 16O5+ current 10Be/9Be ratio
weighted mean ratiowith 1σ−error of mean
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
67
5.4 Results and Discussion of first Measurements
In order to test this new Accelerator SIMS method, three samples each were taken
from ferromanganese crusts ROM96 and GMAT 14D and two samples were taken
from crust D11-1. The 10Be/9Be ratios at various depths in crusts GMAT 14D and
D11-1 have previously been measured with the ISOLAB-120 SIMS instrument in
Oxford (Ling et al., 1997; Frank et al., 1999). The 10Be/9Be ratio as a function of
depth in crust ROM96 was derived by individually measuring the absolute 10Be and 9Be concentrations in the crust with AMS and ICP-MS respectively, and then
calculating the corresponding ratios. Figures 31, 32, and 33 show the results of the
original measurements connected with straight lines together with the results of the
Accelerator SIMS measurements. The horizontal bars through the data points indicate
the depth range out of which the corresponding sample was taken. The vertical error
bars represent the 1σ-error of the 10Be/9Be ratio of the sample, which, in the case of
the ISOLAB-120 measurements, is counting statistics convoluted with an empirically
determined external error. A possible external error in the Accelerator SIMS
measurements is too small to be quantified with the amount of data available from a
single wafer. But because Accelerator SIMS measures ratios normalised to a standard
on the same wafer, the reproducibility between wafers holding the same sample is not
an issue. The error of 3% in the nominal value of the M3 standard was not included
into the errors.
Belshaw et al., 1995, report a discrepancy between the published ratio of the
M3 standard and the ratio measured with the ISOLAB-120 when calibrating to the
NIST SRM-951 boron standard. To take this into account all ratios measured with
Accelerator SIMS were normalised to the ratio of the M3 standard measured with
ISOLAB-120 (3.09⋅10-8) (Belshaw et al., 1995, table 1).
As can be seen from figures 32 and 33 the Accelerator SIMS measurements fit the
data previously measured with ISOLAB-120 very well, apart from two exceptions
(CM13 and CM14) in crust GMAT 14D which will be discussed later on. Especially,
for the sample, for which a direct comparison of the ratio is possible from the depth
range out of which it was taken, the Accelerator SIMS measurement yielded an
identical result to the ISOLAB-120 measurement to within statistics. This is true for
the other samples as well and is quite astonishing, because the crusts were literally
resampled in comparison to taking an aliquot from a reservoir of already sampled
material. All measured ratios and errors are listed in tables in Appendix B. A
complete list of the ratios measured with ISOLAB-120 and the combination of 10Be AMS and ICP-MS for the three crusts is given there as well.
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
68
1
10
100
0 2 4 6 8 10 12 14 16 18 20
Depth [mm]
CM17
CM16
10B
e/9 B
e ra
tio
(10-9
)
Ling et al. (1997)
Fig. 32: 10Be/9Be ratio as a function of depth in ferromanganese crust D11-1 comparing the samples measured with Accelerator SIMS (CM16 and CM17) with data published by (Ling et al., 1997).
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60
Depth (mm)
10B
e/9 B
e ra
tio (
10-9
)
Frank et al. (1999)
CM13
CM14
CM15
Fig. 33: 10Be/9Be ratio as a function of depth in ferromanganese crust GMAT 14D comparing the samples measured with Accelerator SIMS (CM13, CM14 and CM15) with data published by (Frank et al., 1999).
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
69
The large sampling ranges in crust D11-1 are only due to the fact that CM16 and
CM17 were not intended for a direct comparison of methods. As can be seen from the
data of the other crusts, taking greater care while sampling results in a better depth
resolution.
Exceptions are the two data points CM13 and CM14 from crust GMAT 14D. These
samples contained less than 50 ng of Be and were too small for reproducible analysis,
i.e. repetitive measurements gave ratios that scattered by more than the internal error
of the individual measurements. The small samples can be explained with the high
growth rate of the GMAT 14D crust. No measurements of the absolute 9Be
concentration in GMAT 14D have been made, but with ~10 mm/Ma it has grown 5-6
times faster than the other crusts which means that the 9Be concentration is about the
same amount less than in the other crusts (Frank, 2003a). This will yield a lot smaller
samples, if roughly the same amount of material is taken from the ferromanganese
crusts. Because of the uncertainty in the 9Be concentration of GMAT 14D it is
expected that the limit to the sample size is smaller than 50 ng of beryllium.
0.1
1
10
100
0 2 4 6 8 10 12 14 16 18 20
Depth (mm)
10B
e/9 B
e ra
tio (
10-9
)
AMS/ICP-MS
CM10
CM11
CM12
Fig. 34: 10Be/9Be ratio as a function of depth in ferromanganese crust ROM96 comparing the samples measured with Accelerator SIMS (CM10, CM11 and CM12) with ratios obtained with 10Be AMS and 9Be ICP-MS. (Frank et al., 2003b)
In figure 34 the Accelerator SIMS results of samples from crust ROM96 are
compared with results gained from the combination of 10Be AMS and ICP-MS.
Whereas the relative variations from data point to data point are identical for both
methods, a systematic deviation of ~22% between the data points of the two methods
is apparent. This is not too surprising, because due to matrix effects during
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
70
measurement, the day-to-day reproducibility of the absolute 9Be concentration with
ICP-MS is about 15%. This introduces a large uncertainty to the measured absolute
ratios, even if the relative variations can be measured accurately. In general, it is also
the ICP-MS measurements that contribute to the large errors of the data points. By
correcting the ratios of one method by 22%, both methods give identical results within
statistical uncertainty. However, it is Accelerator SIMS that produces the more
reliable results for both absolute ratios as well as relative variations between data
points.
Generally, it can also be said that the precision of the Accelerator SIMS
measurements is better than those of the ISOLAB-120 measurements as well as the
combination of 10Be AMS and ICP-MS.
5.5 Conclusions
First measurements of natural 10Be/9Be ratios with Accelerator SIMS have been
performed successfully, and the analysed total sample sizes of about 100 ng of Be
represent a unique achievement in the field of AMS. It demonstrates that samples of
~1016 atoms in total can be measured reliably and even isotopic ratios of 10-10 can be
measured to a precision of ~10%, higher ratios can be measured to a precision of
~3%. The good agreement of the Accelerator SIMS results with results from other
methods demonstrate that this new method is ready to be used for routine
measurements of carrier-free 10Be samples.
In addition, the expectations to develop a more precise and more sensitive method for
the measurement of natural 10Be/9Be ratios were fulfilled. This means that growth
rates and therefore time scales of ferromanganese crusts can be measured to greater
precision. The measurement of smaller ratios (10-10) in comparison to the method
based on SIMS (10-9) also means that the period, over which the time scales are
measured, can be increased from ~10 Ma to ~15 Ma (3 half-lives of 10Be). For ratios
of 10-10, which are either not measurable (SIMS) or the precision of the measurements
is bad (AMS/ICP-MS), the higher precision of Accelerator SIMS makes the
extrapolation of the time scale to deeper layers of the crust, where 10Be/9Be ratios
cannot be measured anymore, more accurate. It is planned to apply the Accelerator
SIMS method to the measurement of 10Be/9Be ratios in ferromanganese crusts from
sea-mounds in the Mediterranean out-flow zone on the Atlantic side of Gibraltar. The
method will be able to give reliable information on how the water circulation through
the sea-passage changed in the past.
Chapter 5 Direct Measurement of Natural 10Be/9Be Ratios
71
It also might be possible to transfer the technique developed with this method to the
measurement of carrier-free samples of other radioisotopes. In the light of the
development of small AMS-facilities with terminal voltages of less than 1 MV the
potential of using a focussed ion beam to analyse samples could be significant (Suter
et al., 2000).
The results of this chapter combined with the results of chapters 3 and 4 also
demonstrate that it is be possible to measure the presence of trace elements in ppb-
concentrations in samples of 100 ng in size. But for the measurement of isotopic ratios
of trace elements to the precision required in the environmental sciences (better than
1%) ppm concentrations should be available.
73
Chapter 6 Comparison with other Analytical Techniques
It remains to compare the measured figures of merit of Accelerator SIMS with those
of other analytical methods. A detailed comparison of all existing Accelerator SIMS
facilities with every single analytical technique in the wide variety of techniques
available would take too long at this point. Therefore, Accelerator SIMS will mainly
be compared with the two state-of-the-art analytical techniques presently available:
SIMS and ICP-MS.
Ideally, the decision on which analytical method is used for an application is made by
choosing the best analytical method for the set problem. However, it is not only
properties such as sensitivities or detection limits of a technique that influence the
decision. Other factors such as sample throughput, sample preparation, availability of
the analytical apparatus, etc. have a large influence on the final choice. These
boundary conditions are unique from application to application and from laboratory to
laboratory. Therefore, the following discussion will only compare quantifiable
properties of the PSI/ETH Accelerator SIMS facility with the capabilities of other
analytical methods.
Possible improvements to Accelerator SIMS will be presented after this comparison.
There are a variety of technical improvements that can still be made in order to
improve performance and sensitivity. A few of them will be discussed.
6.1 Detection Limit and Lateral Resolution
For bulk concentration analysis of trace elements it is usually the detection limit for a
given lateral resolution that determines the potential of a technique for applications. A
comparison between methods can therefore be made by creating a graph in which
trace element concentration is plotted against analytical spot size. In this graph every
technique is represented by an area that is defined by its detection limits for a given
lateral resolution. Figure 35 shows such a graph with various areas corresponding to
some techniques. Additional techniques that only allow imaging or concentration
analysis are plotted along the corresponding axis. The figure is based on a website of
the Charles Evans and Associates analytical group (Charles Evans and Associates,
2000).
Chapter 6 Comparison with other Analytical Techniques
74
The abbreviations used in figure 35 are the following: Atomic Force Microscopy
(AFM), Scanning Electron Microscopy (SEM), Field Emission Scanning Electron
Microscopy (FE-SEM), Transmission Electron Microscopy (TEM), Auger Electron
Spectrometry (AES), Focussed Electron Beam Auger Electron Spectrometry (FE-
AES), Energy Dispersive X-ray Spectrometry (EDS), µ-Raman Spectroscopy
(Raman), X-Ray Photoelectron Spectroscopy (XPS), Electron Spectroscopy for
Chemical Analysis (ESCA), Fourier Transform Infrared Spectrometry (FTIR), X-Ray
Fluorescence (XRF), Rutherford Backscattering Spectrometry (RBS), Total
Reflection X-Ray Fluorescence (TXRF), Time-of-Flight Secondary Ion Mass
Spectrometry (TOF-SIMS), Secondary Ion Mass Spectrometry (Dynamic-SIMS),
Inductively Coupled Plasma Mass Spectrometry (ICP-MS), Laser Ablation ICP-MS
(LA-ICP-MS), Gas Chromatography/Mass Spectrometry (GCMS), Glow Discharge
Mass Spectrometry (GDMS), Neutron Activation Analysis (NAA).
Fig. 35: Detection limits against lateral resolution: A comparison of regimes accessible to various analytical methods.
Chapter 6 Comparison with other Analytical Techniques
75
The area in which Accelerator SIMS can operate is shown in figure 35 as well. It is
based on results of measurements performed at the PSI/ETH Accelerator SIMS
facility. In addition, the area gained by extrapolating detection limits to expected
values, after the sensitivity of the apparatus had been improved, has been included as
well. As can be seen, the area is basically the regime represented by conventional
SIMS extended towards lower detection limits by more than one order of magnitude.
This was to be expected, since Accelerator SIMS has the advantage of suppressing
molecular and isobaric interferences with the additional use of an accelerator mass
spectrometer. Accelerator SIMS does not rival the high lateral resolutions achievable
with SIMS due to the fact that no state-of-the-art SIMS source has ever been attached
to an accelerator mass spectrometer yet. However, when looking at figure 35 the
following should be kept mind: in chapter 3 it was demonstrated that the sensitivity of
Accelerator SIMS for bulk concentration analysis of a given trace element can vary by
orders of magnitude according to the secondary ion yields of the different elements in
different sample matrices. This feature is not unique to Accelerator SIMS. Other
methods suffer from the same or a similar disadvantage and in some cases a trace
element cannot be analysed with a given technique at all. Figure 35 should therefore
not be interpreted as showing the detection limits for all trace elements in all samples
for a certain lateral resolution, but more as the envelope defined by the sum of
convenient analytical conditions. The decision of which technique is suitable for an
application can therefore not be made based on figure 35 alone.
6.2 Analysis of Small Samples
As was seen in chapter 5, the challenge of analysing small samples lies in the art of
mounting the samples in a way that they can be analysed reproducibly. However, the
following discussion is not going to focus on this aspect. A more general view of
aspects to be considered for such analyses will be given; be it for bulk concentration
analysis or isotopic ratio measurements. Again, a detailed comparison of all possible
techniques would take too long at this point and the following discussion is by no
means exhausting.
Assuming that a sample can be mounted in a way that reproducible measurement is
possible, then the smallest sample size analysable with a given application is
determined by the overall useful yield of the analytical technique under consideration,
or, in other words, on the fraction of atoms that are not detected during analysis and
are therefore lost. A graphical visualisation of such losses suffered by different
Chapter 6 Comparison with other Analytical Techniques
76
techniques is attempted in figure 36 where Accelerator SIMS is compared with SIMS
and ICP-MS. The example used for discussion is the measurement of carrier-free 10Be
samples presented in chapter 5.
It is assumed that a sample of given size - in this case 100 ng of beryllium (100 ng Be
= 6.6⋅1015 Be atoms) with a rare isotope abundance of 10-10 (in this case 10Be with
6.6⋅105 atoms) - is prepared for the analysis in the best possible way. For SIMS the
main losses will then be due to the low ion yield in the sputter process. In this
example, SIMS analyses positive beryllium ions, which have a yield of about 1%
when sputtered from pure beryllium oxide (Benninghoven et al., 1989). The
transmission probability through the mass spectrometer is above 90% and losses are
therefore negligible. This is a feature that Accelerator SIMS has in common with
SIMS. By coincidence, the yield of negative beryllium oxide ions is also about 1%.
However, for Accelerator SIMS additional losses in the intensity of the ion beam are
suffered by additionally sending the ion beam through the accelerator mass
spectrometer. For the specific example being discussed the transmission of 10Be3+ is
15%. Generally, the transmission can range from 0.1% for heavy elements through a
spectrometer with no special design up to 50% and higher for lighter elements through
a dedicated system (Jacob et al., 2000).
1
100
104
106
108
1010
1012
1014
1016
Num
ber
of a
tom
s
Accelerator SIMS SIMS ICP-MS
9Be
Ionisation Yield
Overall Useful Yield
Detector Background
Molecular/IsobaricBackground
9Be 9Be
10Be10Be
10Be
Dynamic Range
Fig. 36: Comparison of the analysis of carrier-free 10Be samples with Accelerator SIMS, SIMS and ICP-MS. The losses in sensitivity due to overall useful yield, detector background, molecular and isobaric interferences and the resulting dynamic range are indicated for the individual techniques.
Chapter 6 Comparison with other Analytical Techniques
77
In figure 36 the fractions of sample atoms lost during analysis are indicated with
horizontal lines for the individual techniques. Ideally, the intensity of the signal of the
rare isotope, which is measured with a technique, should then be proportional to the
remaining number of atoms that make it through the analytical instrument. However,
detector background or interferences that could not be completely suppressed can
reduce the sensitivity of the method. The combination of the overall useful yield and
the different backgrounds that are present define the lower limit of the dynamic range
in which an analytical technique can detect a rare isotope.
ICP-MS cannot be compared as directly to Accelerator SIMS and SIMS, because the
analytical process is different. The sample is usually dissolved in a weak acid. This
solution is then transported to a so-called nebuliser where it is sprayed into a carrier
gas (usually argon). Generally, the salinity of the solution injected into the nebuliser is
not allowed to be higher than 10 mg/ml (or 10 mg/g) otherwise the nebuliser will clog
(Knüsel, 1999). The sample is then carried to the plasma where only a few percent of
the argon flow is injected into the plasma (Beauchemin, 2002). The analysis of
beryllium with ICP-MS is additionally hampered by the presence of 10B+ and 40Ar4+ in
the mass spectrum. These interferences cannot be resolved completely and even for
thoroughly purified samples the detection limit of 10Be has been found to be about
1 ng/g (Knüsel, 1999). Using a hot plasma all the atoms entering the plasma are
ionised and almost all molecules will be destroyed, so 9BeH+ is not a problem.
However, only a fraction of the ions produced by the plasma can be accepted by the
mass spectrometer. Especially, if a magnetic sector field spectrometer with a high
mass resolution is used the overall useful yield will suffer and in cases can be as low
as 10-6 which is not in favour of the analysis of small samples (Beauchemin, 2002;
Turner et al., 2000).
If the salinity of the sample solution is not allowed to be higher than 10 mg/ml then 10Be/9Be ratios of 10-7 are the smallest ratios that can be measured with ICP-MS.
However, this is the highest ratio ever found in the environment (von Blanckenburg et
al., 1996a). Without wanting to perform tedious isotopic enrichment of 10Be in the
sample, ICP-MS is therefore not suitable for this application as can be seen in
figure 36 as well.
To do ICP-MS justice, however, it should be said that the overall useful yield depends
strongly on the type of transport system used to inject the sample into the plasma and
on the type of mass spectrometer used to analyse the ions. In some cases detection
limits of a few fg/g have been achieved (Winefordner et al., 2000; Günther et al.,
1999; Beauchemin, 2002). These are, however, instrumental backgrounds and should
not be confused with the general performance of ICP-MS. The real advantage of ICP-
Chapter 6 Comparison with other Analytical Techniques
78
MS lies in the analysis of vacuum incompatible samples. For example, with Laser
Ablation ICP-MS fluid inclusions in minerals can be analysed (Halter et al., 2002).
The upper limit of the dynamic range would have to be reduced by at least two orders
of magnitude in figure 36, if the bulk concentration of a trace element were to be
analysed instead of an isotopic ratio. If the trace element concentration rises above 1%
then its secondary ion current is no longer proportional to its concentration
(Benninghoven et al., 1989). If the trace element is present in such high abundance
then a calibration curve of secondary ion current versus trace element concentration
would have to be established first, a method that would introduce additional
uncertainties to the measured results. In addition, the matrix and trace elements will
have different ion yields which has an additional influence on the overall useful yields
and therefore on the dynamic range as well.
The combined results presented in this thesis demonstrate that it is possible to
measure the presence of trace elements in ppb concentrations in samples of 100 ng in
size. But for the measurement of isotopic ratios of trace elements to the precision
required in the environmental sciences (better than 1%) ppm concentrations should be
available.
6.3 Improvements to Accelerator SIMS
The full potential of Accelerator SIMS has not been exploited yet. However, further
technical improvements to the existing facility or the design of a new apparatus have
to be made for this. Higher precision and an extension of the capabilities of
Accelerator SIMS would be the result.
By extending the variety of detectors available at the PSI/ETH AMS facility, the
beam bouncing system of the low-energy magnet could be used for quasi-
simultaneous injection of different masses. This would be of advantage for the
measurement of both isotopic ratios of trace elements as well as trace element
concentrations. Because of the possibility to monitor the transmission during
measurement or the possibility to quasi-simultaneously detect different isotopes of a
trace element, the results of Accelerator SIMS measurements would be a lot more
precise and would also extend the capabilities of Accelerator SIMS to analytical
regimes, which are not accessible to other techniques.
For example, the transmission is not monitored during the measurement of platinum
group elements and corrections for a possible variation cannot be made. The main
reason for this is that measurements are presently limited to the use of only one
Chapter 6 Comparison with other Analytical Techniques
79
detector and the measurement of ion currents in Faraday cups. Because the trace
elements analysed are a lot heavier than the main compounds of the sample matrix,
the bouncing system of the low-energy magnet cannot bridge the gap in order to
measure an ion current related to the sample matrix in a Faraday cup. However, a
possible candidate to monitor a beam related to the matrix would be the Cs6+ fragment
coming from the break-up of an intense molecule with a mass close to the mass of the
trace element of interest, for example, the 133Cs28Si2 molecule. The injection mass of
189 amu would lie within the reach of the bouncing system when the low-energy
magnet is tuned to the mass of one of the heavy platinum group elements. In order to
be able to monitor the Cs6+ beam at the position of the detector additional bouncing of
the high-energy electrostatic deflector would be required. This, however, has already
been implemented and does not impose a technical problem (Suter et al., 1984). A
greater challenge lies in the detection system. The field value of the high-energy
magnet has to remain constant during measurement and the resulting separation of the
Cs6+ beam and, for example, the Au9+ beam at the position of the detector would be
~40 mm. With either two small detectors or with a position-sensitive gas ionisation
detector with a broad entrance window (~76 mm) both elements could be monitored
quasi-simultaneously. The same principle could also be used to measure isotopic
ratios of heavy trace elements, for example the 191Ir/193Ir ratio.
A different constraint presently imposed on Accelerator SIMS is that it has only been
performed at large accelerator facilities and accordingly measurements are more
complex and more expensive. To make the method more accessible to scientists
wanting to use Accelerator SIMS, a smaller instrument would have to be developed
that could fit into a regular laboratory. Recent development and commercial success
of small AMS facilities for radiocarbon dating (Suter et al., 1999; Jacob, 2001) and
results obtained for the measurements of other radioisotopes on a small AMS facility
(Stocker et al., 2003) indicate that a versatile compact AMS facility for the
measurement of a wide variety of radioisotopes with similar sensitivities and precision
as on large facilities can be designed. Table 8 summarises the present transmissions
and background levels of radioisotopes measured with the compact 600 kV PSI/ETH
AMS facility. The extremely high transmission of heavy elements such as uranium,
plutonium and thorium in charge state 3+ (15%) is striking (Fifield et al., 2003) and is
almost two orders of magnitude higher than the transmission of similar elements
through the accelerator mass spectrometer used for this thesis. However, generally the
transmission would increase by a factor of 2 to 5. In addition, at the lower energies
increasing background in the detector hampers measurements.
Chapter 6 Comparison with other Analytical Techniques
80
The results of this thesis and the insight gained with the design of compact AMS
facilities suggest that the design of a small compact Accelerator SIMS instrument
operating at low energies is feasible. The compact and with it simpler design of such a
machine would also improve the precision of Accelerator SIMS measurements. The
great distances covered by the ion beam as it travels through the high-energy mass
spectrometer would be shortened considerably making the system less vulnerable to
instabilities of all kinds. The higher transmission through a compact system means
that generally the overall useful yields would improve relative to the accelerator mass
spectrometer used for the measurements presented in this thesis. Correspondingly,
assuming that no additional background problems arise, the detection limits would
improve as well. First test measurements investigating the feasibility of trace element
analysis with stable trace elements are being made at the compact 600 kV PSI/ETH
AMS facility at present (von Wartburg, 2003).
Isotope Charge state Energy [keV] Transmission [%] Background ratio /
Detection limit
10Be 2+ 1500 13 [9Be] 10-13
14C 1+ 970 40 – 45 5⋅10-15
26Al 3+ 2455 9 7⋅10-14
41Ca 1+, 2+, 3+ 1000 - 2000 15 – 20 10-12
129I 4+ 2500 4 10-12
Th 3+ 1240 15 -
Pu 3+ 1200 10 - 15 106 atoms
Table 8: Charge state, resulting energy after acceleration, transmission and background levels for radioisotopes evaluated at the 600 kV PSI/ETH AMS facility. Taken from (Stocker et al., 2003)
Another advantage of a compact Accelerator SIMS facility in comparison to sector
field mass spectrometers is that Accelerator SIMS only requires the use of
spectrometers with a relatively low mass resolution in comparison to state-of-the-art
sector field spectrometers. This means that the ion optics of the system are simpler
and the instrument would be easier to operate and easier to maintain than a high-
resolution sector field spectrometer. The most delicate part of the system would then
be the primary ion source.
The disadvantage of only being able to analyse negative secondary ions could be
compensated with the use of charge-changing canals. In this arrangement positive
Chapter 6 Comparison with other Analytical Techniques
81
secondary ions would be extracted from the sample and passed through an alkali
vapour placed between the low-energy magnet and the tandem accelerator. In this
charge-changing canal the secondary ions would be converted either to neutrals or to
negative ions with an efficiency of, in cases, more than 50% (Heinemeier et al.,
1978a+b; Litherland et al., 1997).
Chapter 6 Comparison with other Analytical Techniques
83
Chapter 7 Final Conclusions
The main goal of this thesis, namely evaluating the potential of Accelerator SIMS
with respect to its application in the environmental sciences, has been met. It can be
said that Accelerator SIMS is superior to any other analytical method available for
specific applications. The applications discussed in chapters 4 and 5 could not have
been performed with the same sensitivity or lateral resolution by any other method
available at present. Therefore the potential of Accelerator SIMS lies in its use for
specific applications in the environmental and other sciences. These can be all sorts of
trace element or isotopic ratio analyses in solids. Applications with radioisotopes
other than 10Be are to be considered as well. The individual achievements are
summarised in the following.
When work towards this thesis commenced, instrumental setups for reproducible
analysis of the trace elements of interest for the applications discussed in this thesis
were non-existent. Isotopes heavier than iodine had never been analysed in trace
element abundances at the PSI/ETH AMS facility before. The facility was technically
upgraded to the extent to make measurements of the kind presented in this thesis
possible. Procedures required for time-efficient start-up of the AMS facility had to be
developed. These tuning procedures can be applied to trace element analysis in the
entire mass range and in different sample matrices. The ability to reliably tune the
PSI/ETH Accelerator SIMS facility is a prerequisite to tackle future applications.
With the procedures developed for the two applications, know-how has been gained
with respect to the sample forms necessary for a reliable Accelerator SIMS analysis
and for quicker tuning of the instrument itself. Future applications are able to
substantially benefit from this.
In addition, the design of the ion optical elements was improved to achieve a better
stability of the secondary ion currents. The transmission through the system could be
increased by a factor of more than ten for all elements, thereby improving the
sensitivities and detection limits by the same factor.
The sensitivities and detection limits of platinum group elements, gold, and silver
(PGE) analysed in a silicon dioxide matrix were studied by implanting PGE into
SIMOX-wafers at two different depths and evaluating the resulting depth profiles
measured with Accelerator SIMS. It was shown that the sensitivity of Accelerator
Chapter 6 Comparison with other Analytical Techniques
84
SIMS drops by nearly two orders of magnitude in comparison to the analysis of PGE
in a silicon matrix. This is mainly due to a reduction in the ionisation probability in
the sputter process. The type of matrix a trace element is analysed in therefore greatly
influences the properties of a measurement.
For the in-situ analysis of PGE in sedimentary layers of the KT-boundary,
additionally a procedure to analyse electrically non-conducting samples had to be
developed. Best results were achieved by covering the samples with a few nanometer
thick electrically conducting layer. Subsequently, in-situ bulk concentrations of less
than 1 ng/g of iridium in sedimentary layers were measured. The lateral resolution of
the measurements was 100 µm. To date, such concentrations have so far only been
measurable with NAA and AMS, methods mainly suitable for bulk concentration
analysis only and without any significant lateral resolution. With Accelerator SIMS it
has therefore become possible to study concentration distributions of trace elements in
the ng/g-range at a lot smaller scale.
A similar situation was encountered when work towards the measurement of carrier-
free 10Be samples commenced. In this case a procedure had to be found in order to
tune the facility to be able to reliably analyse the asthmatic currents of a few hundred
pA extracted from 100 ng-samples (total sample size) and to measure their isotopic
ratios of 10-10 to a precision of ~10%. Analysis of such small samples has never been
performed in the field of AMS before and the greatest challenge was to find an
adequate way to load the samples onto wafers so that, on one hand, they did not drop
off the wafers and on the other that their geometrical shape enabled reproducible
measurement. The developed procedure has been explained in detail in chapter 5. Test
measurements gave a good agreement with results obtained with already existing
methods and showed that the new Accelerator SIMS method measures more precisely
than the other methods. This means that growth rates and therefore time scales of
ferromanganese crusts can be measured to greater precision. The measurement of
smaller ratios (10-10) in comparison to the method based on SIMS (10-9) also means
that the period over which the time scales are measured can be increased from ~10 Ma
to ~15 Ma (3 half-lives of 10Be). For ratios of 10-10, where the precision of the
previously available methods gets worse, the higher precision of Accelerator SIMS
makes the extrapolation of the time scale to deeper layers of the crust, where 10Be/9Be
ratios cannot be measured anymore, more accurate.
With the two applications discussed here, is has been demonstrated that Accelerator
SIMS has unique properties and can perform measurements that cannot be performed
with any other analytical technique. However, the technique can still be made more
flexible and more powerful. As described in section 6.3 extensions made to the
Chapter 7 Final Conclusions
85
available detection systems at the PSI/ETH AMS facility would make the
measurement of isotopic ratios of trace elements possible and the precision of results
would be improved by monitoring transmission during measurement. Charge
changing canals could compensate the restriction to only being able to analyse
negative secondary ions.
In order to make Accelerator SIMS more competitive to other state-of-the-art
analytical methods, the size of the apparatus has to be reduced. Recent success of
compact AMS facilities suggests that Accelerator SIMS is possible at low energies as
well ( < 2 MeV ). In this case Accelerator SIMS would also profit from the higher
transmissions that have been achieved with such facilities and which would result in
higher sensitivities and better detection limits. Mass spectrometers with only a
relatively low mass resolution would have to be used for a compact Accelerator SIMS
facility, making the system simpler and operation easier than with sophisticated sector
field spectrometers. This way Accelerator SIMS would become more accessible to
scientists in all fields of research and would develop to be a more powerful tool than it
already is.
87
Appendix A Beryllium Chemistry
The recipe below is the one used for chemical preconcentration and purification of
beryllium from ferromanganese crusts. Sample material was removed from the crust
by sawing a rut into a cross section of the crust with a fine saw. The dust collected
from this process was fed into the chemical purification process.
Sample dissolution Dissolve ~100 mg of Fe-Mn-crust in 1.3 ml 6M HCl at low heat (7 ml Savillex Vial) Transfer the solution to a centrifuge tube leaving the coarse residue in the vial Centrifuge at 10‘000 Rpm for 5 Min. Transfer solution to a 15 ml centrifuge tube pH 8 precipitation with NH3
Centrifuge and discard supernate Dissolve precipitate in 1 ml 6M ∆HCl
Column Fe 2ml Biorad (Dowex) 1x8 100-200 mesh in small Biorad Column stored in 1M ∆HCl Open column and let storage liquid drop out 2ml Millipore H2O: clean resin 2ml 1M ∆HCl: clean resin 5ml + 5ml 6M ∆HCl: Condition resin Load sample (1ml 6M HCl) Collect Be in 7 ml Savillex beaker 0.5ml + 0.5ml + 1ml + 2ml: 6M ∆HCl Collect Be in 7 ml Savillex beaker 2ml 1M ∆HCl: clean resin 2ml Millipore H2O: clean resin 2ml 1M ∆HCl: clean resin Seal and store columns in 1M ∆HCl Evaporate samples Dissolve precipitate in 2 ml Oxalic acid Column Be (Small Column B) 1 ml Biorad (Dowex) AG50-X8 (200-400 mesh) in small Biorad Column stored and
sealed in 6M HCl Open column and let acid drop out 2 ml ~3M ∆HCl: clean resin 10 ml 6M ∆HCl: clean resin 2 ml ~3M ∆HCl: clean resin
Appendix A Beryllium Chemistry
88
3 ml + 2 ml Millipore H2O: Remove HCl from resin 3 ml + 2 ml: 0.4M Oxalic Acid: Condition resin Load sample (in 2 ml Oxalic acid) 1 ml + 1 ml + 2 ml 0.4M Oxalic Acid: Wash down sample and elute Fe, Al, Ti, etc. 1 ml + 2 ml Millipore H2O: Remove Oxalic Acid from Column 10 ml 0.5M ∆HCl: Elute Na 2 ml 1M ∆HCl 6 ml 1M ∆HCl Collect Be in 7 ml Savillex beaker ~ 2 ml ~3M ∆HCl: clean resin 5 ml 6M ∆HCl clean resin Seal & store column in ~6M ∆HCl Evaporate samples AG MG 50 (Small Column A) Dissolve sample in 1 ml 0.4M Oxalic Acid 1ml Biorad AG MP-50 in small Biorad Column stored in H2O Open column and let storage liquid drop out 1 ml 3M ∆HCl: clean resin 10 ml 6M ∆HCl: clean resin 1 ml 3M ∆HCl: clean resin 1 ml H2O: clean resin 1 ml 3M ∆HCl: clean resin 10 ml 6M ∆HCl: clean resin 1 ml 3M ∆HCl: clean resin 2 ml H2O: clean resin 2ml + 2ml 0.4M Oxalic Acid: Condition resin Load sample in 1 ml 0.4M Oxalic Acid 0.5ml + 0.5ml 0.4M Oxalic Acid: Wash sample down 4ml + 4ml 0.4M Oxalic Acid: Elute Al, Fe, Ti, REE, etc. 2ml + 2ml H2O: Wash Oxalic Acid out 3 ml 1M ∆HCl: wash 5 ml 1M ∆HCl: Collect Be (in cleaned Savillex beaker) 1 ml 3M ∆HCl: clean resin 6 ml 6M ∆HCl: clean resin 2 ml 3M ∆HCl: clean resin 2 ml 1M ∆HCl: clean resin Seal and store columns in 1M ∆HCl Evaporate samples
89
Appendix B Results of the Carrier-free 10Be Measurements
The following tables summarise mean 10Be/9Be ratios, <R>, and the corresponding
errors of the mean ratios, σR, of the carrier free beryllium samples CM10 to CM17 as
measured with Accelerator SIMS and presented in chapter 5. If a direct comparison to
a published ISOLAB or AMS/ICP-MS measurement is possible these figures are
given as well (Ling et al., 1997; Frank et al., 1999; Frank et al., 2003b).
The mean ratios are error-weighted means of three to five individual measurements.
Errors of the individual measurements are usually dominated by counting statistics.
Weighting the measurements with their errors is therefore similar to weighting them
with the integrated charge of the 16O5+ current collected during the corresponding
measurement. This is justified considering that in some measurements the samples
hardly gave any current at all. The definitions of the individual parameters are given
below.
Sample Accelerator SIMS ISOLAB or AMS/ICP-MS <R>
[⋅10-9] σR [%]
σR [⋅10-9]
<R> [⋅10-9]
σR [%]
σR [⋅10-9]
CM10 35.2 2.25 0.8 - - -
CM11 6.15 3.55 0.2 4.8 19 0.9
CM12 1.21 5.82 0.1 0.7 100 0.7
CM13 100 2.71 2.7 - - -
CM14 108 2.97 3.2 80.5 4.22 3.4
CM15 31.8 3.49 1.1 32.3 5.26 1.7
CM16 78.2 3.22 2.52 - - -
CM17 13.8 4.29 0.59 - - - Table 9: Measured 10Be/9Be ratios with their corresponding errors.
If N is the number of 10Be/9Be ratio measurements, Ri, of a given sample with the 1σ-
errors, σi , then the weighted mean ratio <R> is calculated with:
∑
∑
=
=
σ
σ>=< N
1i2i
i
N
1i2i
1
R1
R (B.1)
Appendix B Results of the carrier-free 10Be-Measurements
90
The error of the weighted mean, σR, is calculated according to
∑= σ
=σ
N
1i2i
2R
11 (B.2)
The table 10 summarises the 10Be/9Be ratios measured in the crusts GMAT 14D,
D11-1 and ROM96 with on one hand the ISOLAB-120 instrument and on the other
the combination of 10Be AMS and ICP-MS. These are the ratios the Accelerator SIMS
measurements were compared to in figures 32, 33 and 34 in chapter 5 and as
published in (Ling et al., 1997; Frank et al., 1999; Frank et al., 2003b).
GMAT 14D (ISOLAB-120) D11-1 (ISOLAB-120)
Depth [mm] <R> [⋅10-9] σR [⋅10-9] Depth [mm] <R> [⋅10-9] σR [⋅10-9]
0-1 115 9.6 0.4-1.0 82.5 3.4
7-8 80.5 3.4 2.7-3.7 57 4.5
14-16 58.8 3.4 5.5-6.3 18.4 1
26-28 32.3 1.7 7.6-8.4 7.48 1.1
36-38 20.9 1.2 10.4-11.5 3.69 0.35
49-51 12.3 0.7 12.6-13.4 2.58 0.27
15.2-16.3 1.49 0.13 61-67 7.0 0.4
19.9-21.1 0.8 0.21
ROM96 (AMS/ICP-MS)
Depth [mm] Ratio [⋅10-9] Error [⋅10-9]
0-0.2 43.0 3.3
2.0-2.5 17.7 1.6
6.0-7.0 4.8 0.9
9.5-10.5 3.1 0.8
14.0-15.0 1.5 0.9
18.0-19.0 0.7 0.67
Table 10: Summary of 10Be/9Be ratios used for comparing Accelerator SIMS measurements to. The data was taken from (Ling et al., 1997; Frank et al., 1999; Frank et al., 2003b).
91
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Thank you! Merci! Muchas Gracias!
With the title of these acknowledgements I can only hint my gratefulness for the help
I have received during work towards this thesis. There are many, many people who
have helped me along the way and I hope that I can repay them sometime with the
same passionate interest in their problems as they helped me with mine. My apologies
to anyone I might have forgotten in the following, but you can be assured that, your
help has been greatly appreciated!
Specifically, I would like to thank Prof. Dr. Martin Suter for welcoming me into his
research group and giving me the opportunity and the necessary in-sight into AMS to
be able to write this thesis. It has been great to work in such a flexible team. I would
also like to thank the Swiss National Science Foundation for providing the necessary
funding for my project.
Of course, a big ‘thank you’ has to go to Prof. Dr. Ralph Eichler, who willingly
accepted to referee this thesis and did so with such thoroughness as is seldom
encountered for a person with such a long list of commitments.
Also, Prof. Dr. Walter Kutschera has to be thanked for refereeing this thesis. It is not
taken for granted and I wish you and your AMS group in Vienna all the best for the
future.
My supervisor, Max Döbeli, was always a great help during times when the going was
rather tough. Thank you, for always having time to listen to me and for your help to
look at things from a different perspective.
I would also like to thank Dr. Beda Hofmann from the Natural History Museum in
Bern for providing the samples from the KT-boundary and Prof. Dr. Rainer Wieler for
the interesting discussions regarding possible applications of Accelerator SIMS and
for providing a fragment of the Allende meteorite for test measurements.
Before I switch to German I would also like to thank all the people I met at
conferences and workshops. This thesis would not have succeeded with out being able
to share your knowledge on the subject.
Den Personen, welche mich am nächsten auf dem Weg durch meine Doktorarbeit
begleitet haben möchte ich aber auf Deutsch danken. Dies sind natürlich Cornelia
Aurelio und Marina Jeanrenaud, die guten Feen des Sekretariats, Georges Bonani
Alexander Duhr, Rainer Mühle, Christoph Schnabel und Hans-Arno Synal mit
Thank You! Merci! Muchas Gracias!
100
welchen ich immer allerlei Probleme und auch anderes diskutieren konnte, Peter
Eberhardt für alle elektronischen Belange, Wolfgang Gruber für die Erledingung von
allerlei Kleinigkeiten, die auch gemacht sein wollten, Paul Hermann für die Hilfe bei
der Reparatur der Ionenquelle, Susan Ivy-Ochs, Irka Hajdas, Gianni Leone und
Fabian Scheifele dafür, dass ich ihre Labors betreten und mitbenutzen durfte, Peter
Kubik für die Hilfe bei der Einstellung der Maschine für die trägerfreien 10Be-
Messungen, und natürlich Rudolf Pfenninger für den PC-Support.
Natürlich sind da auch meine Mitdoktoranden, denen ich für die gute Zeit und gute
Kameradschaft danken möchte: Stefan Jacob, Silvio Tschudi, Jürgen Scheer, Henning
Fuhrmann, Philipp Gartenmann, Michal Grajcar, Christian Kottler und Martin
Stocker. Ich wünsche allen viel Erfolg für ihre Zukunft!
Ein ganz spezielles Dankschön geht aber an unsere Operateure: Peter Kägi, der dafür
gesorgt hat, den Beschleuniger bei Laune zu halten, mit dem ich aber stets mit viel
Elan über allerlei Sachen diskutieren konnte. Peter, danke dafür, dass Du mir immer
gezeigt hast, was zu einem guten Schweizer gehört! (Fussball spielen sie aber
deswegen leider doch nicht besser…) Jürg Thut, deine technisch einfachen Lösungen
zu schwierigeren Problemen waren immer willkommen. Vor allem aber deine guten
Tipps wenn es um Motorradreparaturen ging waren unersetzlich! Und René Gruber,
danke für die Hilfe bei der Reparatur von allerlei Sachen und für die präzise
Umsetzung aller Pläne, die ich Dir je gebracht habe, auch wenn es jeweils am Freitag
Nachmittag um 3 Uhr war, draussen schönes Wetter und Dein Motorrad ungeduldig
vor dem Haus wartete. Gute Fahrt wünsche ich Dir weiterhin!
Meine Geologenfreunde vom ETH Zentrum dürfen auch nicht vergessen gehen:
Martin Frank und Tina van der Flierdt, danke für die Einweihung in die Geheimnisse
der Probenaufbereitung von Mangankrusten, der Hilfe beim Vergleich meiner
Resultate mit anderen Methoden und für die gute Atmosphäre in eurem Labor. Es war
sicherlich eine ernüchternde Erfahrung für mich als Physiker. Ich hoffe ihr hattet aber
genauso Spass an den schönen Regenbogenfarben meiner Säulen. Felix Oberli, danke
für die Organisation von Öfen, die ich benutzen konnte wenn mal die 10Be/9Be-
Verhältnisse meiner Proben so hoch lagen, dass ich Zugangsverbot zu unserer
Infrastruktur bekam.
Und zu guter letzt haben natürlich auch viele gute Seelen im Hintergrund gewirkt,
welche mich privat immer unterstützt und ermutigt haben oder auch für eine Kletter-
oder Skitour oder aber auch für ein Bier zu haben waren, wenn ich mal eine Pause
brauchte. Vielen Dank!
101
Curriculum Vitae
Name Colin Maden
Parents David and Kari Fimland Maden-Fozzard
Born May 4th, 1973 in Oxford (GB)
Citizen of Riniken (AG) and
Great Britain (holder of double-citizenship)
Education:
1980 – 1985 Elementary school in Brugg
1985 – 1989 Bezirksschule Brugg
1989 – 1993 Alte Kantonsschule Aarau, Matura Typus C
1993 – 1998 Study of Physics at ETH Zurich with specialisation in nuclear physics
Diploma thesis under the supervision of Prof. Dr. J. Lang and
Prof. Dr. M. Suter with the title: Analyse von metallischen
Spurenelementen in Silizium mit Beschleuniger-Sekundärionen-
Massenspektrometrie
1995/1996 ERASMUS exchange year at the University of Bergen
1998 – 2003 Doctoral studies at the institute of particle physics at ETH Zurich in
the research group of Prof. Dr. M. Suter
February 2003 Presentation of this thesis
Professional Experience:
July–Oct. ‘97 First Level Support for Enterprise Network Services, regions
Switzerland, Germany, and Austria, Sun Microsystems, Zurich.
1998 – 2003 Research assistant in the group of ion beam physics at ETH Zurich and
Paul Scherrer Institute. Activities: Assistant in research and lecturing,
measurement of radioisotopes with Accelerator Mass Spectrometry,
administration of UNIX systems.